Enforce keys for all structs

lib/music_brainz/config.ex

@@ -4,6 +4,7 @@ defmodule MusicBrainz.Config do
           user_agent: String.t()
         }
 
+  @enforce_keys [:api, :user_agent]
   defstruct api: MusicBrainz.APIImpl,
             user_agent: "change me"
 

lib/music_brainz/release_group.ex

@@ -1,4 +1,5 @@
 defmodule MusicBrainz.ReleaseGroup do
+  @enforce_keys [:id, :type, :title, :artists, :release]
   defstruct [:id, :type, :title, :artists, :release]
 
   def from_api_response(rg) do