Add basic OK response
Convert response.ok to fixed value
Fix typo in passthrough handler
Convert method_not_allowed response to value

This commit converts `response.method_not_allowed` from a function
that returns a response (tuple) to just the response tuple itself.
This'll keep things simpler for people, especially to differentiate
handlers (as functions) from responses (as values).
Check for bad methods in method_router

Prior to this commit, if you used a `routers.method_router`,
and it got passed a request with an HTTP method not specified
in the method_router routes, you'd get a server exception.

This commit makes it so that instead of a getting a server exception
on an unknown HTTP method, instead you get HTTP 405 Method Not Allowed.