bpo-29412: Fix indexError when parsing a header value ending unexpectedly (GH-14387) (GH-14411)

* patched string index out of range error in get_word function of _header_value_parser.py and created tests in test__header_value_parser.py for CFWS.
* Raise HeaderParseError instead of continuing when parsing a word.
(cherry picked from commit 7213df7bbfd85378c6e42e1ac63144d5974bdcf6)

Co-authored-by: Abhilash Raj <maxking@users.noreply.github.com>
3 files changed