Add Permissions attribute to User

Update integration tests to ensure Permissions field is populated
Add Accept header for this functionality, so it works on GHE 2.5
Fix impacted URL unit tests

Octokit.Tests.Integration/Reactive/ObservableRepositoryCollaboratorClientTests.cs

@@ -26,6 +26,8 @@ public class ObservableRepositoryCollaboratorClientTests
                 var collaborators = await fixture.GetAll(context.RepositoryOwner, context.RepositoryName).ToList();
                 Assert.NotNull(collaborators);
                 Assert.Equal(2, collaborators.Count);
+                Assert.NotNull(collaborators[0].Permissions);
+                Assert.NotNull(collaborators[1].Permissions);
             }
         }