Add tests to verify current observable behavior

Right now, our observable methods are semi-cold. They don't make the
request until subscribed, but multiple subscriptions get the same
result. This is the way that Octokit.cocoa works.

Octokit.Tests.Integration/Reactive/ObservableRepositoriesClientTests.cs

@@ -0,0 +1,33 @@
+using System.Reactive.Linq;
+using System.Threading.Tasks;
+using Octokit.Reactive.Clients;
+using Xunit;
+
+namespace Octokit.Tests.Integration
+{
+    public class ObservableRepositoriesClientTests
+    {
+        public class TheGetMethod
+        {
+            [IntegrationTest]
+            public async Task ReturnsSpecifiedRepository()
+            {
+                var github = new GitHubClient("Octokit Test Runner")
+                {
+                    Credentials = Helper.Credentials
+                };
+                var client = new ObservableRepositoriesClient(github);
+                var observable = client.Get("haacked", "seegit");
+                var repository = await observable;
+                var repository2 = await observable;
+
+                Assert.Equal("https://github.com/Haacked/SeeGit.git", repository.CloneUrl);
+                Assert.False(repository.Private);
+                Assert.False(repository.Fork);
+                Assert.Equal("https://github.com/Haacked/SeeGit.git", repository2.CloneUrl);
+                Assert.False(repository2.Private);
+                Assert.False(repository2.Fork);
+            }
+        }
+    }
+}