Make MailHeaderMap perform case-insensitive searches

The trait MailHeaderMap has two methods for searching headers, normal
and case-insensitive. This documentation says:

> According to the spec the mail headers are supposed to be
> case-sensitive, but in real-world scenarios that's not always the
> case.

Actually, the spec says nothing that headers supposed to case-sensitive,
and historical precedent is to be case-insensitive. Certainly, all the
mail processing software I am familiar with (Thunderbird, qmail, ezmlm,
and mutt) treat headers as case insensitive. RFC 822 section 3.4.7 says:

> When matching any other syntactic unit, case is to be ignored. For
> example, the field-names "From", "FROM", "from", and even "FroM" are
> semantically equal and should all be treated identically.

While RFC 2822 did drop that section (and RFC 5322 didn't restore it),
others interpret other parts to indicate case insensitivity.

https://stackoverflow.com/a/6143644

> RFC 5322 does actually specify this, but it is very indirect.

https://www.gnu.org/software/emacs/manual/html_node/emacs/Mail-Headers.html

> Upper and lower case are equivalent in field names.

https://nifi.apache.org/docs/nifi-docs/components/org.apache.nifi.processors.email.ExtractEmailHeaders/index.html

> NOTE the header key is case insensitive

This change makes all header searches case insensitive.

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