Ensure that the SparkRateLimitError.retry_after attribute is always an int

Fix for issue #52.  The retry-after header (when provided) is a string and should be cast to an integer for obvious reasons.  😄

Author: cmlccie

ciscosparkapi/exceptions.py

@@ -145,7 +145,7 @@ def __init__(self, response):
         super(SparkRateLimitError, self).__init__(response)
 
         # Extended exception data attributes
-        self.retry_after = response.headers.get('Retry-After', 200)
+        self.retry_after = int(response.headers.get('Retry-After', 200))
         """The `Retry-After` time period (in seconds) provided by Cisco Spark.
 
         Defaults to 200 seconds if the response `Retry-After` header isn't