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/preview_spec.rb

@@ -59,9 +59,16 @@ describe Onebox::Preview do
   end
 
   describe "#engine" do
+    let(:preview_image_url) { "http://www.example.com/image/without/file_extension" }
+    let(:preview_image) { described_class.new(preview_image_url, content_type: 'image/png') }
+
     it "returns an engine" do
       expect(preview.send(:engine)).to be_an(Onebox::Engine)
     end
+
+    it "can match based on content_type" do
+      expect(preview_image.send(:engine)).to be_an(Onebox::Engine::ImageOnebox)
+    end
   end
 
   describe "xss" do