Add default for OAuth 2.0 parameter access_type of offline.

Reviewed in http://codereview.appspot.com/5316073/.
diff --git a/oauth2client/client.py b/oauth2client/client.py
index 3d81a8c..d77e776 100644
--- a/oauth2client/client.py
+++ b/oauth2client/client.py
@@ -641,7 +641,10 @@
     self.user_agent = user_agent
     self.auth_uri = auth_uri
     self.token_uri = token_uri
-    self.params = kwargs
+    self.params = {
+        'access_type': 'offline',
+        }
+    self.params.update(kwargs)
     self.redirect_uri = None
 
   def step1_get_authorize_url(self, redirect_uri='oob'):