Be more tolerant when parsing RFC2047

* src/rfc2047parse.c (G_MIME_RFC2047_WORKAROUNDS): Enable.

With enabling the workarounds we try to treat everything between
the =? ?= markers of a word as an encoded word and don't bail
if there are not correctly encoded chaacters in there.
Enigmail does not encode dots in filenames. GMimes understanding
of the RFC differes about that point.