add convention test to ensure all response models have a public parameterless ctor

2 model classes were missing one - technically not needed for these 2 classes due to their api calls being basic data types that then get populated into these objects, but it's easiest to just add them to these 2 classes so the test can pass on all response models

Octokit/Models/Response/RepositoryLanguage.cs

@@ -7,6 +7,9 @@ namespace Octokit
     [DebuggerDisplay("{DebuggerDisplay,nq}")]
     public class RepositoryLanguage
     {
+        public RepositoryLanguage()
+        { }
+
         public RepositoryLanguage(string name, long numberOfBytes)
         {
             Name = name;