Returns a 405 instead of a 404 on GET /{diagramType} and GET /{diagramName}/{outputFormat}

[ggrossetie]

New users are sometimes confused with the 404 status code. For instance, GET /mermaid and GET /mermaid/svg currently returns a 404 because the method GET is not supported (i.e., it expects a POST request with an application/json or text/plain payload).

The server must generate an Allow header field in a 405 status code response. The field must contain a list of methods that the target resource currently supports.

In this case, the Allow header should be:

Allow: POST

Similarly, we should make sure that we return a 415 Unsupported Media Type if the user tries to send a request with an unsupported format (such as application/xml).
But we can address this concern in an other issue/pull request.