[bug] The default MethodNotAllowedHandler is not spec compliant

[tebruno99]

MIGRATED
From mux created by rocketspacer: gorilla#554

RFC 7231 Section 6.5.5

States that

The origin server MUST generate an
Allow header field in a 405 response containing a list of the target
resource's currently supported methods.

Which is clearly not implemented right now
https://github.com/gorilla/mux/blob/75dcda0896e109a2a22c9315bca3bb21b87b2ba5/mux.go#L601-L603

Suggestion:

• Rather than assign a default MethodNotAllowedHandler just reuse the NotFoundHandler so it's both backward compatible and spec-compliant.

[tebruno99]

@elithrar Would you prefer a flag to skip handling MethodNotAllowed or removing the default MethodNotAllowed handler altogether? (meaning: MethodNotAllowed will only be handled if the handler is explicitly specified)

Or even something like ExplicitMethodNotAllowedHandler to flag the 2nd proposal.

[tebruno99]

See comments in gorilla#555