fix: set default POST policy scheme to "http" (#172)

Author: Gurov Ilya

google/cloud/storage/client.py

@@ -928,7 +928,7 @@ def generate_signed_post_policy_v4(
         credentials=None,
         virtual_hosted_style=False,
         bucket_bound_hostname=None,
-        scheme=None,
+        scheme="http",
         service_account_email=None,
         access_token=None,
     ):