Improve header encoding with folding

[tisdall]

rebased replacement of #209

Changes proposed in this pull request

We should fold content to try to keep within 78 characters per line (excluding CRLF). This PR keeps ASCII as ASCII, but folds at foldable whitespace. Also, reworks encoding-words to properly encoding spaces as "_" and to leave foldable space between blocks.

• restructured encode_header_value:
  • to drop :quoted_printable and accept the header name (to help support folding on first line)
  • if encoding is needed:
    • put into blocks of up to 75 characters with space between to allow folding (previously all the encoding blocks were just put together with no space or CRLF between them)
    • make spaces into _ to prevent folding within an encoded word (and to follow the RFC)
• do "folding" as a follow-up step as it quickly becomes confusing with email address fields that should be folded after commas and not just on any space
  • currently only does folding on "other" fields that aren't matched by more specific render_header_value/2 functions

I reviewed folding for DKIM headers, and I believe this handles those properly as it also requires folding on "foldable whitespace"