Tags: KarpelesLab/gomailparse

Add DecodeHeader helper for RFC 2047 header decoding

Export DecodeHeader as a package-level function and add
Part.DecodedHeader(key) as a convenience shorthand for
DecodeHeader(p.Header.Get(key)). Raw header values in
Part.Header remain untouched.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>