ApiV1Controller: Fix missing : in render_404
ApiV1Controller: Add missing "code" field in render_error and render_errors
Change ScrobblesTest to actually assert the requests
Test that scrobbling returns 404 if user is not found
ScrobblesController: Remove needless "respond_to" blocks
ScrobblesController: Move user fetching into a private method and memoize result
ScrobblesController: Move "track" fetching into a memoizing getter
Remove unused request mocks from SongsTest
Song: Use songs_file in refresh method