Code cleanup for Attatchment::saveAs()
Fix Attachment encoding problem when saving to disk

The case statement returned true for 0 == 'quoted-printable',
and base64 decoded all 7bit encoded attachments.
I have removed the string representations as an int is always
returned by PHP's imap functions for encoding type.
Merge pull request #82 from Xethron/fix-attachment-encoding

Fix attachment encoding bug introduced in 6f09bdc2