[imap] decode header values in RFC 2047 encoded-word format

RFC 2047 outlines an encoding for header fields.  Without this
change the header fields (To, From, Subject, etc.) may have
values like `=?iso-8859-1?Q?abc?=` instead of `abc`.
Merge pull request #790 from bfritz/imap-decode-header-values

[imap] decode header values in RFC 2047 encoded-word format