Check for "Re:" at start of subject in case insensitive manner

To avoid a subject ending up like:

    "Re: RE: RE: RE: original subject"

That I've seen a few times.

The fix is to lower case the subject and the strings to compare against
before calling startwith()
Merge pull request #660 from foobacca/case-insensitive-reply-prefix

Check for "Re:" at start of subject in case insensitive manner