Implement renaming and deprecation of GitHub App Installation Access …

…Token route (#1860)

* change existing CreateInstallationToken method to use new /app/installations route, and provide an obsolete CreateInstallationTokenPreview() method still using the old route, in order to support GHE 2.14

* suppress obsolete usage warning in tests

* add unit test for new method

* add new method to observable client

* fix routes in tests

* remove CreateInstallationTokenPreview() as it isnt actually required

Octokit.Tests/Clients/GitHubAppsClientTests.cs

@@ -98,7 +98,7 @@ public void PostsToCorrectUrl()
 
                 client.CreateInstallationToken(fakeInstallationId);
 
-                connection.Received().Post<AccessToken>(Arg.Is<Uri>(u => u.ToString() == "installations/3141/access_tokens"), string.Empty, "application/vnd.github.machine-man-preview+json");
+                connection.Received().Post<AccessToken>(Arg.Is<Uri>(u => u.ToString() == "app/installations/3141/access_tokens"), string.Empty, "application/vnd.github.machine-man-preview+json");
             }
         }
     }

Octokit/Helpers/ApiUrls.cs

@@ -287,7 +287,7 @@ public static Uri NotificationSubscription(int id)
         /// <param name="installationId">The Id of the GitHub App installation.</param>
         public static Uri AccessTokens(long installationId)
         {
-            return "installations/{0}/access_tokens".FormatUri(installationId);
+            return "app/installations/{0}/access_tokens".FormatUri(installationId);
         }
 
         /// <summary>