ensure datetime offset is encoded correctly for search APIs (octokit#…

…2091)

* add failing integration test for timestamp

* make the test pass by encoding the value

* generalize this pattern for now

* update tests to reflect change in date formatting

Octokit.Tests.Integration/Clients/SearchClientTests.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using Octokit;
@@ -23,6 +24,18 @@ public async Task SearchForCSharpRepositories()
         Assert.NotEmpty(repos.Items);
     }
 
+    [IntegrationTest]
+    public async Task SearchForCSharpRepositoriesUpdatedIn2020()
+    {
+        var request = new SearchRepositoriesRequest("csharp")
+        {
+            Updated = DateRange.GreaterThan(new DateTimeOffset(2020, 1, 1, 0, 0, 0, TimeSpan.Zero))
+        };
+        var repos = await _gitHubClient.Search.SearchRepo(request);
+
+        Assert.NotEmpty(repos.Items);
+    }
+
     [IntegrationTest]
     public async Task SearchForForkedRepositories()
     {