[FEATURE] Ignore case sensitivity when matching defined methods

Is there an existing feature request for this?

• I have searched the existing feature requests

Is your feature request related to a problem? Please describe.

If a client is (mis)configured to make requests with lowercase method names, it is impossible to use gorilla/mux to match.

Describe the solution that you would like.

Case insensitive matches on methods.

Describe alternatives you have considered.

One way is to stop strings.ToUpper-ing defined methods, but that would break compatibility. Documenting the current limitation if the behavior is not changed would also go far.

Anything else?

A test case that demonstrates the condition:

func TestLowercaseMethods(t *testing.T) {
	r := mux.NewRouter()

	r.HandleFunc("/hc", func(w http.ResponseWriter, r *http.Request) {
		w.WriteHeader(http.StatusOK)
	}).Methods("get", "GET")

	methods := []string{"get", "GET"}

	for i := range methods {
		w := httptest.NewRecorder()
		r.ServeHTTP(w, httptest.NewRequest(methods[i], "/hc", nil))
		t.Logf("Method %q: got %d", methods[i], w.Result().StatusCode)
	}
}

// Method "get": got 405
// Method "GET": got 200

Also, thanks!