parser: Tune stringifyMapKeys

```bash
benchmark                                               old ns/op     new ns/op     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     3269          3053          -6.61%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        4.79          5.23          +9.19%
BenchmarkStringifyMapKeysIntegers-4                     2707          2320          -14.30%

benchmark                                               old allocs     new allocs     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     16             6              -62.50%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0              0              +0.00%
BenchmarkStringifyMapKeysIntegers-4                     16             6              -62.50%

benchmark                                               old bytes     new bytes     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     1080          1008          -6.67%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0             0             +0.00%
BenchmarkStringifyMapKeysIntegers-4                     1080          1008          -6.67%
```

2 files changed, 50 insertions, 16 deletions

diff --git a/parser/frontmatter.go b/parser/frontmatter.go
index c5aaf807a..664cb6d5c 100644
--- a/parser/frontmatter.go
+++ b/parser/frontmatter.go
@@ -23,6 +23,8 @@ import (
 	"io"
 	"strings"
 
+	"github.com/spf13/cast"
+
 	"github.com/BurntSushi/toml"
 	"github.com/chaseadamsio/goorgeous"
 
@@ -209,7 +211,9 @@ func HandleYAMLMetaData(datum []byte) (map[string]interface{}, error) {
 	// gotten from `json`.
 	if err == nil {
 		for k, v := range m {
-			m[k] = stringifyMapKeys(v)
+			if vv, changed := stringifyMapKeys(v); changed {
+				m[k] = vv
+			}
 		}
 	}
 
@@ -221,16 +225,19 @@ func HandleYAMLMetaData(datum []byte) (map[string]interface{}, error) {
 func HandleYAMLData(datum []byte) (interface{}, error) {
 	var m interface{}
 	err := yaml.Unmarshal(datum, &m)
+	if err != nil {
+		return nil, err
+	}
 
 	// To support boolean keys, the `yaml` package unmarshals maps to
 	// map[interface{}]interface{}. Here we recurse through the result
 	// and change all maps to map[string]interface{} like we would've
 	// gotten from `json`.
-	if err == nil {
-		m = stringifyMapKeys(m)
+	if mm, changed := stringifyMapKeys(m); changed {
+		return mm, nil
 	}
 
-	return m, err
+	return m, nil
 }
 
 // stringifyMapKeys recurses into in and changes all instances of
@@ -239,23 +246,31 @@ func HandleYAMLData(datum []byte) (interface{}, error) {
 // described here: https://github.com/go-yaml/yaml/issues/139
 //
 // Inspired by https://github.com/stripe/stripe-mock, MIT licensed
-func stringifyMapKeys(in interface{}) interface{} {
+func stringifyMapKeys(in interface{}) (interface{}, bool) {
 	switch in := in.(type) {
 	case []interface{}:
-		res := make([]interface{}, len(in))
 		for i, v := range in {
-			res[i] = stringifyMapKeys(v)
+			if vv, replaced := stringifyMapKeys(v); replaced {
+				in[i] = vv
+			}
 		}
-		return res
 	case map[interface{}]interface{}:
 		res := make(map[string]interface{})
 		for k, v := range in {
-			res[fmt.Sprintf("%v", k)] = stringifyMapKeys(v)
+			ks, err := cast.ToStringE(k)
+			if err != nil {
+				ks = fmt.Sprintf("%v", k)
+			}
+			if vv, replaced := stringifyMapKeys(v); replaced {
+				res[ks] = vv
+			} else {
+				res[ks] = v
+			}
 		}
-		return res
-	default:
-		return in
+		return res, true
 	}
+
+	return nil, false
 }
 
 // HandleJSONMetaData unmarshals JSON-encoded datum and returns a Go interface
diff --git a/parser/frontmatter_test.go b/parser/frontmatter_test.go
index 4d28facb2..7281ca336 100644
--- a/parser/frontmatter_test.go
+++ b/parser/frontmatter_test.go
@@ -323,37 +323,56 @@ func TestRemoveTOMLIdentifier(t *testing.T) {
 
 func TestStringifyYAMLMapKeys(t *testing.T) {
 	cases := []struct {
-		input interface{}
-		want  map[string]interface{}
+		input    interface{}
+		want     interface{}
+		replaced bool
 	}{
 		{
 			map[interface{}]interface{}{"a": 1, "b": 2},
 			map[string]interface{}{"a": 1, "b": 2},
+			true,
 		},
 		{
 			map[interface{}]interface{}{"a": []interface{}{1, map[interface{}]interface{}{"b": 2}}},
 			map[string]interface{}{"a": []interface{}{1, map[string]interface{}{"b": 2}}},
+			true,
 		},
 		{
 			map[interface{}]interface{}{true: 1, "b": false},
 			map[string]interface{}{"true": 1, "b": false},
+			true,
 		},
 		{
 			map[interface{}]interface{}{1: "a", 2: "b"},
 			map[string]interface{}{"1": "a", "2": "b"},
+			true,
 		},
 		{
 			map[interface{}]interface{}{"a": map[interface{}]interface{}{"b": 1}},
 			map[string]interface{}{"a": map[string]interface{}{"b": 1}},
+			true,
 		},
 		{
 			map[string]interface{}{"a": map[string]interface{}{"b": 1}},
 			map[string]interface{}{"a": map[string]interface{}{"b": 1}},
+			false,
+		},
+		{
+			[]interface{}{map[interface{}]interface{}{1: "a", 2: "b"}},
+			[]interface{}{map[string]interface{}{"1": "a", "2": "b"}},
+			false,
 		},
 	}
 
 	for i, c := range cases {
-		res := stringifyMapKeys(c.input)
+		res, replaced := stringifyMapKeys(c.input)
+
+		if c.replaced != replaced {
+			t.Fatalf("[%d] Replaced mismatch: %t", i, replaced)
+		}
+		if !c.replaced {
+			res = c.input
+		}
 		if !reflect.DeepEqual(res, c.want) {
 			t.Errorf("[%d] given %q\nwant: %q\n got: %q", i, c.input, c.want, res)
 		}
@@ -407,7 +426,7 @@ func BenchmarkStringifyMapKeysStringsOnlyStringMaps(b *testing.B) {
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		stringifyYAMLMapKeys(m)
+		stringifyMapKeys(m)
 	}
 }