Invalidate cached responses after POST

[marcelmaatkamp]

CacheControlAdapter.invalidating_methods lists PUT, PATCH and DELETE, but not POST.
A successful POST therefore leaves a previously cached GET for the same URL in place, and the
next GET is served from the cache with the pre-POST body.

RFC 9111 §4.4 requires invalidation on any unsafe method and names POST in the first sentence:

Because unsafe request methods (Section 9.2.1 of [HTTP]) such as PUT, POST, or DELETE have the
potential for changing state on the origin server, intervening caches are required to invalidate
stored responses to keep their contents up to date.

Reproduction

Against a server whose GET body changes after each POST:

GET, POST, GET  ->  value=0 then value=0    # stale
GET, PUT,  GET  ->  value=0 then value=1    # correct

Same server, same handler, same headers; only the method differs.

The change

One entry added to invalidating_methods, plus a test.

The three existing tests (test_put_invalidates_cache and friends) assert not r2.from_cache on
the response to the unsafe request itself, which is never served from cache regardless. The new
test asserts on the GET that follows, so it fails without the fix:

FAILED tests/test_adapter.py::TestSessionActions::test_post_invalidates_cache[use_adapter]
FAILED tests/test_adapter.py::TestSessionActions::test_post_invalidates_cache[use_wrapper]

With the change, the full suite passes (105 tests).

Scope

This does not implement the MAY part of §4.4 (invalidating URIs from Location and
Content-Location), nor the "methods whose safety is unknown" clause. Both are separate changes.