added tests to catch regression

Octokit.Tests/Models/SearchUsersRequestTests.cs

@@ -1,4 +1,5 @@
-using Octokit;
+using System;
+using Octokit;
 using Octokit.Tests.Helpers;
 using Xunit;
 
@@ -16,5 +17,13 @@ internal class SearchUsersRequestTests
             // If I can cast this to a writeable collection, then that defeats the purpose of a read only.
             AssertEx.IsReadOnlyCollection<string>(result);
         }
+
+        [Fact]
+        public void SortNotSpecifiedByDefault()
+        {
+            var request = new SearchUsersRequest("shiftkey");
+            Assert.True(String.IsNullOrWhiteSpace(request.Sort));
+            Assert.False(request.Parameters.ContainsKey("sort"));
+        }
     }
 }