Make MainClass.get_app return completed GithubApp when slug is given (#2543)

Fixes slug attribute to fetch app when not set. Loads app with slug lazily, fetch complete app without slug.

github/GithubApp.py

@@ -109,6 +109,7 @@ class GithubApp(github.GithubObject.CompletableGithubObject):
         """
         :type: string
         """
+        self._completeIfNotSet(self._slug)
         return self._slug.value
 
     @property