added some fixes

Octokit.Reactive/Clients/ObservableMergingClient.cs

@@ -25,6 +25,10 @@ namespace Octokit.Reactive
         /// <returns></returns>
         public IObservable<Merge> Create(string owner, string name, NewMerge merge)
         {
+            Ensure.ArgumentNotNullOrEmptyString(owner, "owner");
+            Ensure.ArgumentNotNullOrEmptyString(name, "name");
+            Ensure.ArgumentNotNull(merge, "merge");
+
             return _client.Create(owner, name, merge).ToObservable();
         }
 
@@ -39,6 +43,8 @@ namespace Octokit.Reactive
         /// <returns></returns>
         public IObservable<Merge> Create(int repositoryId, NewMerge merge)
         {
+            Ensure.ArgumentNotNull(merge, "merge");
+
             return _client.Create(repositoryId, merge).ToObservable();
         }
     }

Octokit.Tests/Reactive/ObservableMergingClientTests.cs

@@ -46,13 +46,15 @@ namespace Octokit.Tests.Clients
                 var client = new ObservableMergingClient(Substitute.For<IGitHubClient>());
 
                 var newMerge = new NewMerge("baseBranch", "shaToMerge") { CommitMessage = "some mergingMessage" };
+
                 Assert.Throws<ArgumentNullException>(() => client.Create(null, "name", newMerge));
                 Assert.Throws<ArgumentNullException>(() => client.Create("owner", null, newMerge));
                 Assert.Throws<ArgumentNullException>(() => client.Create("owner", "name", null));
+
                 Assert.Throws<ArgumentNullException>(() => client.Create(1, null));
+
                 Assert.Throws<ArgumentException>(() => client.Create("", "name", newMerge));
                 Assert.Throws<ArgumentException>(() => client.Create("owner", "", newMerge));
-                Assert.Throws<ArgumentException>(() => client.Create("owner", "", null));
             }
         }