HTTP 405 response bodies/headers cannot be customised

[atheriel]

Example application or steps to reproduce the problem

Every Plumber API sent an incorrect method to a valid route (e.g. PATCH /route where only GET /route is supported) is affected by this issue.

Describe the problem in detail

Plumber has pr_set_404() and pr_set_error() functions (among other, older approaches) that allow users to control the response body/headers of errors returned from an API. We use this extensively to emit common error structures. However, since #746 the HTTP 405 response has been hardcoded internally and it is no longer possible to control it.

This is not a bug per se (since it is intended behaviour) but it is a regression, since it was previously possible to control HTTP 405 responses from the 404 handler.

I originally proposed in #493 that Plumber have a dedicated handler for HTTP 405 responses (presumably pr_set_405()), and still believe that that is the right level of control here -- I would be happy to implement this myself, too.