Add GCloud parametersas kwargs for storage.url

It enables using kwargs, such as `content_type` https://cloud.google.com/python/docs/reference/storage/latest/google.cloud.storage.blob.Blob#google_cloud_storage_blob_Blob_generate_signed_url

storages/backends/gcloud.py

@@ -297,7 +297,7 @@ def get_created_time(self, name):
         created = blob.time_created
         return created if setting('USE_TZ') else timezone.make_naive(created)
 
-    def url(self, name):
+    def url(self, name, parameters=None):
         """
         Return public url or a signed url for the Blob.
         This DOES NOT check for existance of Blob - that makes codes too slow
@@ -309,6 +309,8 @@ def url(self, name):
         no_signed_url = (
             blob_params.get('acl', self.default_acl) == 'publicRead' or not self.querystring_auth)
 
+        params = parameters or {}
+
         if not self.custom_endpoint and no_signed_url:
             return blob.public_url
         elif no_signed_url:
@@ -318,13 +320,14 @@ def url(self, name):
             )
         elif not self.custom_endpoint:
             return blob.generate_signed_url(
-                expiration=self.expiration, version="v4"
+                expiration=self.expiration, version="v4", **params
             )
         else:
             return blob.generate_signed_url(
                 bucket_bound_hostname=self.custom_endpoint,
                 expiration=self.expiration,
                 version="v4",
+                **params
             )
 
     def get_available_name(self, name, max_length=None):