Text for 405 errors is a lie

[richvdh]

When you send a GET request to an endpoint that only supports POST, or vice versa, synapse sends back an error that looks like:

Method Not Allowed

Your browser approached me (at /_matrix/saml2/authn_response) with the method "GET". I only allow the method HEAD here.

We hardly support HEAD anywhere, and it doesn't say anything about POST.

(This actually only happens on a subset of endpoints, such as the media repo ones. For others we give back a 400, which I think is fine).

Related: #6746

[clokep]

If we wanted to fix this with our async resources we essentially need the logic from twisted.web.resource._computeAllowedMethods at the bottom of our _AsyncResource._async_render method.

[LutzLange]

I just hit this error. I was using python requests and did not notice that it was using a GET request instead of a POST request.