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

app/controllers/well_known/webfinger_controller.rb

@@ -8,8 +8,13 @@ module WellKnown
       @magic_key = pem_to_magic_key(@account.keypair.public_key)
 
       respond_to do |format|
-        format.xml  { render content_type: 'application/xrd+xml' }
-        format.json { render content_type: 'application/jrd+json' }
+        format.any(:json, :html) do
+          render formats: :json, content_type: 'application/jrd+json'
+        end
+
+        format.xml do
+          render content_type: 'application/xrd+xml'
+        end
       end
     rescue ActiveRecord::RecordNotFound
       head 404