[3.13] gh-144156: Fix email header folding concatenating encoded words (GH-144692...

[3.13] gh-144156: Fix email header folding concatenating encoded words (GH-144692) (#145195)

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)
(cherry picked from commit 0f7cd5544a4dd1d7cf892c93c661510d619caaa7)

Co-authored-by: Robsdedude <dev@rouvenbauer.de>
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>