Organization membership preview API changes - Review user permission (#1633)

* Add organization membership preview header

* Add API endpoints to preview a collaborators permission

* Add methods to preview a collaborators permission

* Add methods to the observable repo collaborator client

* Fix convention test failure

* Use correct API endpoint when using repository ID

* Move the helper function pair so they aren't in between another pair

* Use the correct URL for the review permission API endpoint

* Add unit tests

* Add integration tests for review permission methods

* Fix spelling mistake

* Renaming enum as per review

Octokit.Reactive/Clients/ObservableRepoCollaboratorsClient.cs

@@ -128,6 +128,41 @@ namespace Octokit.Reactive
             return _client.IsCollaborator(repositoryId, user).ToObservable();
         }
 
+        /// <summary>
+        /// Review a user's permission level in a repository
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://developer.github.com/v3/repos/collaborators/#review-a-users-permission-level">API documentation</a> for more information.
+        /// </remarks>
+        /// <param name="owner">The owner of the repository</param>
+        /// <param name="name">The name of the repository</param>
+        /// <param name="user">Username of the collaborator to check permission for</param>
+        /// <exception cref="ApiException">Thrown when a general API error occurs.</exception>
+        public IObservable<CollaboratorPermission> ReviewPermission(string owner, string name, string user)
+        {
+            Ensure.ArgumentNotNullOrEmptyString(owner, "owner");
+            Ensure.ArgumentNotNullOrEmptyString(name, "name");
+            Ensure.ArgumentNotNullOrEmptyString(user, "user");
+
+            return _client.ReviewPermission(owner, name, user).ToObservable();
+        }
+
+        /// <summary>
+        /// Review a user's permission level in a repository
+        /// </summary>
+        /// <remarks>
+        /// See the <a href="https://developer.github.com/v3/repos/collaborators/#review-a-users-permission-level">API documentation</a> for more information.
+        /// </remarks>
+        /// <param name="repositoryId">The id of the repository</param>
+        /// <param name="user">Username of the collaborator to check permission for</param>
+        /// <exception cref="ApiException">Thrown when a general API error occurs.</exception>
+        public IObservable<CollaboratorPermission> ReviewPermission(long repositoryId, string user)
+        {
+            Ensure.ArgumentNotNullOrEmptyString(user, "user");
+
+            return _client.ReviewPermission(repositoryId, user).ToObservable();
+        }
+
         /// <summary>
         /// Adds a new collaborator to the repository.
         /// </summary>