Updated Label subscription field to work with project and group API

Fixed a bug were group label were returning subscription false even
on subscribed labels.
Duplicated the logic from labels_helper.rb label_subscription_status()
Changelog: fixed
Signed-off-by: Lucas Zampieri <lzampier@redhat.com>

lib/api/entities/label.rb

@@ -18,7 +18,12 @@ module API
       end
 
       expose :subscribed do |label, options|
-        label.subscribed?(options[:current_user], options[:parent])
+        if label.subscribed?(options[:current_user]) ||
+          label.subscribed?(options[:current_user], options[:parent])
+          true
+        else
+          false
+        end
       end
     end
   end