Modify oauth2client.multistore_file to store and retrieve credentials using an arbitrary key.

Reviewed in https://codereview.appspot.com/7067054/.
diff --git a/apiclient/http.py b/apiclient/http.py
index 28dba64..bfb7f77 100644
--- a/apiclient/http.py
+++ b/apiclient/http.py
@@ -1194,7 +1194,7 @@
 
     Args:
       http: httplib2.Http, an http object to be used in place of the one the
-        HttpRequest request object was constructed with.  If one isn't supplied
+        HttpRequest request object was constructed with. If one isn't supplied
         then use a http object from the requests in this batch.
 
     Returns: