Comment 2 for bug 1525954

The issue is with RFC 2047 encoded Subject: and the fact that white space is insignificant between RFC 2047 "encoded words".

This fix adds a trailing space to the subject_prefix (from which it has been previously stripped) under what I think is just those circumstances where it is needed.

There is another issue in that under some circumstances, the prefix will be followed by two spaces. This is unchanged by this fix and I have no plan to try to change it. I think this only occurs when the prefix is ascii only and the incoming Subject: already contains the prefix, but doesn't contain a Re: indicator.