Fix the bogus '=' at the end of some quoted-printable messages.

Trailing CRLF is significant in Quoted-Printable transfer encoding.
Stripping trailing whitespace corrupts the encoding, resulting in
odd '=' chars showing up at the end of decoded emails. Fixed by only
stripping leading whitespace.

Closes #440
Merge pull request #479 from jeremy/fix-quoted-printable-equals-sign

Fix trailing equals signs mysteriously showing up on quoted-printable emails