A minor change for ProviderDiscovery and spec (#3543)

* Do not default the format in ProviderDiscovery

The format should be determined when discovering, as it is in the current
implementation, and it is a flaw if it is not determined.

* Spec ProviderDiscovery

app/lib/provider_discovery.rb

@@ -31,7 +31,7 @@ class ProviderDiscovery < OEmbed::ProviderDiscovery
         raise OEmbed::NotFound, url
       end
 
-      OEmbed::Provider.new(provider_endpoint, format || OEmbed::Formatter.default)
+      OEmbed::Provider.new(provider_endpoint, format)
     end
   end
 end