Fix serialization error with null PushedAt

It's possible for a repository to have a null value for
the "pushed_at" field. For example, if it's been created
online and never pushed to via git.

Octokit.Tests.Integration/RepositoriesClientTests.cs

@@ -1,4 +1,5 @@
-using System.Threading.Tasks;
+using System;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace Octokit.Tests.Integration
@@ -22,6 +23,23 @@ namespace Octokit.Tests.Integration
                 Assert.False(repository.Fork);
             }
 
+            [IntegrationTest]
+            public async Task ReturnsNeverPushedRepository()
+            {
+                var github = new GitHubClient
+                {
+                    Credentials = AutomationSettings.Current.GitHubCredentials
+                };
+
+                var repository = await github.Repository.Get("Test-Octowin", "PrivateTestRepository");
+
+                Assert.Equal("https://github.com/Test-Octowin/PrivateTestRepository.git", repository.CloneUrl);
+                Assert.True(repository.Private);
+                Assert.False(repository.Fork);
+                Assert.Equal(3709146, repository.Id);
+                Assert.Null(repository.PushedAt);
+            }
+
             [IntegrationTest]
             public async Task ReturnsForkedRepository()
             {

Octokit/GitHubModels.cs

@@ -432,7 +432,7 @@ namespace Octokit
         public int WatchersCount { get; set; }
         public string MasterBranch { get; set; }
         public int OpenIssuesCount { get; set; }
-        public DateTimeOffset PushedAt { get; set; }
+        public DateTimeOffset? PushedAt { get; set; }
         public DateTimeOffset CreatedAt { get; set; }
         public DateTimeOffset UpdatedAt { get; set; }