Update outdated test.

The current test is asserting against an outdated version of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.