Encoding trimming

[SebastianZach]

I received a mail with:

Content-Type: text/html; charset="UTF-8" 
Content-Transfer-Encoding: quoted-printable\n\n
X-Spam-Checker-Version: ... 

This lead to unhandled encoding error in encoding.go

[chaoticryptidz]

Can reproduce.

[emersion]

I'd just say this e-mail is too broken.