Bugfix: describe_event_bus()['Policy'] should be JSON string, not object

boto3.client('events')['Policy'] must be a string as per:
http://boto3.readthedocs.io/en/latest/reference/services/events.html#CloudWatchEvents.Client.describe_event_bus

Adding json.dumps() around the policy value ensures we do not return an unexpected dict

This change corrects an error when attempting to decode the policy:

json.load(boto3.client('events').describe_event_bus()['Policy'])

moto/events/models.py

@@ -1,5 +1,6 @@
 import os
 import re
+import json
 
 from moto.core.exceptions import JsonRESTError
 from moto.core import BaseBackend, BaseModel
@@ -238,8 +239,10 @@ class EventsBackend(BaseBackend):
                 'Action': 'events:{0}'.format(data['action']),
                 'Resource': arn
             })
+        policy = {'Version': '2012-10-17', 'Statement': statements}
+        policy_json = json.dumps(policy)
         return {
-            'Policy': {'Version': '2012-10-17', 'Statement': statements},
+            'Policy': policy_json,
             'Name': 'default',
             'Arn': arn
         }