Added '7bit' to possible body content-transfer-encodings, treating it th...

...e same as quoted-printable

Hi andris,
Here is the second of my pull requests (the first was for your encoding library). It adds a line to decode Content-Transfer-Encoding: 7bit as quoted-printable when finalizing body text. I don't know if this is the right way to do it, but I had some Japanese emails sent from iPhones that wouldn't get decoded because 7bit wasn't a possible case in _finalizeContents.
It's a one-line change, and I hope you can make use of it!