Add self.retry_after to SparkRateLimitError class

ciscosparkapi/exceptions.py

@@ -140,4 +140,12 @@ def __init__(self, response):
 
 class SparkRateLimitError(SparkApiError):
     """Cisco Spark Rate-Limit exceeded Error."""
-    pass
+
+    def __init__(self, response):
+        assert isinstance(response, requests.Response)
+
+        # Set a sane default just incase Spark lets us down
+        self.retry_after = 200
+
+        if 'Retry-After' in response.headers.keys():
+            self.retry_after = int(response.headers['Retry-After'])

ciscosparkapi/restsession.py

@@ -281,7 +281,7 @@ def request(self, method, url, erc, **kwargs):
                 # Wait and retry if automatic rate-limit handling is enabled
                 if self.wait_on_rate_limit and e.retry_after:
                     logger.info("Received rate-limit message; "
-                                "waiting {:0.0f} seconds."
+                                "waiting {0} seconds."
                                 "".format(e.retry_after))
                     time.sleep(e.retry_after)
                     continue