Failing test demonstrating incorrect MIME word decoding with quotes

Extra quotes appear around MIME words containing special chars, such as
a comma.  See [rt3 #23813].
Avoid adding too many quotes around decoded MIME words

As demonstrated by the tests in the previous commit.

This case, and much more, is handled better by
4.2-on-4.0/protect-more-chars-while-decoding-headers, but that branch is
destined for 4.2.  For now, simply check the leading and trailing
snippets around the matched word for quotes.

See [rt3 #23813].
Merge branch '4.0/mimeword-decoding-quotes' into 4.0-trunk