Update outdated test.

The current test is asserting against an outdated version of
Request#method where HEAD requests are treated as GET requests.
Update test to clearly reflect what it is testing for.
Fix duplicated test.

There is already another test covering Request#request_method. This
test should cover Request#method.
Remove redundant test.

Since we're stubbing the request, the test is actually just asserting
that `@method = env['REQUEST_METHOD']`. In order to the test against
the methodoverride middleware, we should test it against an actual
request. However, Rack is already covering this scenario so we can
remove this test.
Merge pull request #16098 from tgxworld/fixes_to_request_test

Fixes to request method test.