Enable PUT to missing to return 404

[ordnungswidrig]

See #120 for the motivation. In some cases it's preferable to have the server return 404 on PUT to a non-existing resource.

Note: in general the semantic of PUT to a missing resource is that the resource will be created under that location. That means it makes sense for the server to answer with "not implemented" if it does not support the creation of the resource. PUT can mean create or update and "not found" is only sensible as response to the update case. But a PUT to a non-existing resource is a create, so "not found" doesn't make sense :-)

Anyways, people seem so require PUT-to-missing to return 404 and we should make this at least configurable. A work-around was sketched in #120 which worked by forcing the status code to 404 in handle-not-implemented.