Use IReadOnlyList instead of IReadOnlyCollection or IEnumerable

For response models.

Octokit/Models/Request/SearchRepositoriesRequest.cs

@@ -101,7 +101,7 @@ namespace Octokit
         /// </summary>
         public DateRange Updated { get; set; }
 
-        public override IReadOnlyCollection<string> MergedQualifiers()
+        public override IReadOnlyList<string> MergedQualifiers()
         {
             var parameters = new List<string>();