Specs: add tests for asset_pipeline.rb (#554)

I enabled settings for test mode, which doesn't seem to noticeably
impact performance, but gives us a little bit more test coverage.

config/asset_pipeline.rb

@@ -22,13 +22,13 @@ def append_paths(app)
   end
 
   def configure_development(app)
-    app.configure :development do
+    app.configure :development, :test do
       app.sprockets.cache = Sprockets::Cache::FileStore.new("./tmp")
     end
   end
 
   def configure_production(app)
-    app.configure :production do
+    app.configure :production, :test do
       app.sprockets.css_compressor = :scss
       app.sprockets.js_compressor = :uglify
     end

spec/config/asset_pipeline_spec.rb

@@ -0,0 +1,9 @@
+require "spec_helper"
+
+describe "AssetPipeline" do
+  it "handles asset requests" do
+    get("/assets/stylesheets/application.css")
+
+    expect(last_response.body).to match("#feed-title")
+  end
+end