Set user agent header for oembed requests

Certain services (like facebook) require a user agent for
oembed requests, now we can easily set one via config 🎉

Author: dsager

lib/article_json/utils/o_embed_resolver/base.rb

@@ -28,7 +28,11 @@ def parsed_api_response
 
         # @return [Hash]
         def http_headers
-          { 'Content-Type' => 'application/json' }
+          headers = { 'Content-Type' => 'application/json' }
+          unless ArticleJSON.configuration.oembed_user_agent.nil?
+            headers['User-Agent'] = ArticleJSON.configuration.oembed_user_agent
+          end
+          headers
         end
 
         class << self

spec/article_json/utlis/o_embed_resolver/vimeo_video_spec.rb

@@ -22,13 +22,22 @@
 
     let(:oembed_response) { File.read('spec/fixtures/vimeo_video_oembed.json') }
     let(:expected_headers) { { 'Content-Type' => 'application/json' } }
+    let(:expected_response) { JSON.parse(oembed_response, symbolize_names: 1) }
 
     before do
       stub_request(:get, expected_oembed_url)
         .with(headers: expected_headers)
         .to_return(body: oembed_response)
     end
 
-    it { should eq JSON.parse(oembed_response, symbolize_names: true) }
+    context 'with no additional headers' do
+      it { should eq expected_response }
+    end
+
+    context 'with additional headers' do
+      before { ArticleJSON.configure { |c| c.oembed_user_agent = 'foobar' } }
+      let(:expected_headers) { super().merge('User-Agent' => 'foobar') }
+      it { should eq expected_response }
+    end
   end
 end