Handle question marks, etc in headers

Handle extra-long emojis in headers

Remove extra line break

Broke parsing in real world clients but not validators. Yay.

Add custom headers

Specify UTF-8 encoding

Fix broken comparison

Don't enforce return path, and check for leading <

Add return-path header

Only use rfc2047 when required

Turns out some recipients don't handle encoded words properly. This was
causing a garbled subject in Freshdesk, so only use it when required.

Handle long subjects in the new encoding style