HTTPHeaders: raise TypeError on non-string keys in set/get/delitem

[HrachShah]

Fixes #3649.

HTTPHeaders.__setitem__, __getitem__, and __delitem__ did not validate the key type before calling _normalize_header, which is @lru_cache-wrapped and only accepts str. Passing a non-string key (e.g. h[1] = 'x') leaked AttributeError: 'int' object has no attribute 'split' from inside the cache. __contains__ already short-circuits the same case with False; this patch makes the three other indexers raise TypeError with a clear message instead, matching the public MutableMapping[str, str] typing.

tests/test/httputil_test.py adds four regression tests covering __setitem__, __getitem__, __delitem__, and __contains__. The first three fail on master with the original AttributeError and pass on the fix; the contains test pins the existing correct behaviour.

python3 -m tornado.test.httputil_test: 60 passed (56 baseline + 4 new). Black-formatted. PR opens on the upstream tornadoweb/tornado fork HrachShah/tornado branch fix/headers-reject-non-string-keys.