gh-144156: Fix email header folding concatenating encoded words (#144692)

gh-144156: Fix email header folding concatenating encoded words (#144692)

The fix for gh-92081 (gh-92281) was unfortunately flawed, and broke whitespace handling for encoded word patterns that had previously been working correctly but had no corresponding tests, unfortunately in a way that made the resulting headers not RFC compliant, in such a way that Yahoo started rejecting the resulting emails.  This fix was released in 3.14 alpha 1, 3.13 beta 2 and 3.12.5.   This PR fixes the original problem in a way that does not break anything, and in fact fixes a small pre-existing bug (a spurious whitespace after the ':' of the header label if the header value is immediately wrapped on to the next line).  (RDM)

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: R. David Murray <rdmurray@bitdance.com>