fix mail reading: don't raise invalid byte sequence in UTF-8 when reading non-UTF-8 emails

recreated the pull request I did in #341 to fix #340. This version should also be fine with ruby 1.8.7 - if not, please adjust.
This changes the encoding of its argument unexpectedly. Compare with `value = value.dup.force_encoding('binary')` or `value = value.encode('binary')`