Rewrite each header names (and other double quoted strings) to fix case insensitivity

[asimpletune]

The RFC5322 spec states

Characters will be specified either by a decimal value
(e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by
a case-insensitive literal value enclosed in quotation marks (e.g.,
"A" for either uppercase or lowercase A).

There's no way to do this in tspeg yet without alternating characters, e.g. [Ff][Oo][Oo] for foo