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

config/routes.rb

@@ -16,7 +16,7 @@ Rails.application.routes.draw do
   end
 
   get '.well-known/host-meta', to: 'well_known/host_meta#show', as: :host_meta, defaults: { format: 'xml' }
-  get '.well-known/webfinger', to: 'well_known/webfinger#show', as: :webfinger, defaults: { format: 'json' }
+  get '.well-known/webfinger', to: 'well_known/webfinger#show', as: :webfinger
 
   devise_for :users, path: 'auth', controllers: {
     sessions:           'auth/sessions',