[3.8] bpo-21315: Fix parsing of encoded words with missing leading ws (GH-13425)...

[3.8] bpo-21315: Fix parsing of encoded words with missing leading ws (GH-13425) (GH-15655)

* [bpo-21315](https://bugs.python.org/issue21315): Fix parsing of encoded words with missing leading ws.

Because of missing leading whitespace, encoded word would get parsed as
unstructured token. This patch fixes that by looking for encoded words when
splitting tokens with whitespace.

Missing trailing whitespace around encoded word now register a defect
instead.

Original patch suggestion by David R. Murray on [bpo-21315](https://bugs.python.org/issue21315).
(cherry picked from commit 66c4f3f38b867d8329b28c032bb907fd1a2f22d2)

Co-authored-by: Abhilash Raj <maxking@users.noreply.github.com>
(cherry picked from commit dc20fc4311dece19488299a7cd11317ffbe4d3c3)

Co-authored-by: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
https://bugs.python.org/issue21315