fix: check if response attribute of WebPushException object is not None in __str__ method (#169)

this properly checks if extra info about Response object should be added
to string representation of WebPushException as the Response object is
falsy in case of the status code being between 400 and 600

Author: 15bubbles

pywebpush/__init__.py

@@ -41,7 +41,7 @@ def __init__(self, message, response=None):
 
     def __str__(self):
         extra = ""
-        if self.response:
+        if self.response is not None:
             try:
                 extra = ", Response {}".format(
                     self.response.text,