Update to using WebPush Authorization type

As per #19

Author: robbiet480

python/py_vapid/__init__.py

@@ -131,5 +131,5 @@ def sign(self, claims, crypto_key=None):
         else:
             crypto_key = pkey
 
-        return {"Authorization": "Bearer " + sig.strip('='),
+        return {"Authorization": "WebPush " + sig.strip('='),
                 "Crypto-Key": crypto_key}