FEATURE: Onebox can match engines based on the content_type (#13876)

* FEATURE: Onebox can match engines based on the content_type

`FinalDestination` now returns the `content_type` of a resolved URL.

`Oneboxer` passes this value to `Onebox` itself. Onebox engines can now specify a `matches_content_type` regex of content_types that the engine can handle, regardless of the URL.

`ImageOnebox` will match URLs with a content type of `image/png`, `jpg`, `gif`, `bmp`, `tif`, etc.

This will allow images that exist at a URL without a file type extension to be correctly rendered, assuming a valid `content_type` is returned.

spec/lib/onebox/engine_spec.rb

@@ -50,6 +50,18 @@ describe Onebox::Engine do
     end
   end
 
+  describe "handles_content_type?" do
+    class OneboxEngineImages
+      include Onebox::Engine
+      @@matcher_content_type = /^image\/png$/
+    end
+
+    it "returns true if argument matches the matcher" do
+      result = OneboxEngineImages.handles_content_type?('image/png')
+      expect(result).to eq(true)
+    end
+  end
+
   class AlwaysHttpsEngineExample < OneboxEngineExample
     always_https
   end