Improve handling of HTTP_ACCEPT for webfinger (#2008)

This change includes:

- Improve the spec coverage for incoming request to the webfinger action
- For requests without an accept header (ie, what a browser might look like),
  return a JSON response.
- For requests with an explicit format of xml or json, return that format.
- For requests using an accept header, return that format.

Also adds failing spec showing webfinger does not return xml, which covers the
issue described in: https://github.com/tootsuite/mastodon/issues/1983

spec/routing/well_known_routes_spec.rb

@@ -10,6 +10,6 @@ end
 describe 'the webfinger route' do
   it 'routes to correct place with json format' do
     expect(get('/.well-known/webfinger')).
-      to route_to('well_known/webfinger#show', format: 'json')
+      to route_to('well_known/webfinger#show')
   end
 end