fixed bug on using su on play level not setting become method correctly

lib/ansible/modules/core

@@ -1 +1 @@
-Subproject commit 34c4e0d4959eeaf5dc4d2b69d2bd435267e8ff91
+Subproject commit 31cc5f543f4166eddb334340fd559765dc6c3940

lib/ansible/playbook/play.py

@@ -172,6 +172,7 @@ class Play(object):
         elif 'su' in ds:
             self.become=True
             self.become=ds['su']
+            self.become_method='su'
             if 'su_user' in ds:
                 self.become_user=ds['su_user']