Implement 405 for unhandled non-GET requests.

[kristijanburnik]

Respond with a 405 Method not allowed for non-GET requests which don't match a handler.

The assumption is that a client is sending a POST/PUT/DELETE request for a resource which is otherwise available via GET, therefore respond with 405.

If a handler is not found and the GET method was used, we assume the resource does not exist (404).

@jgraham

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/5586

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