fix: allow metadata keys to be cleared

Metadata keys can be cleared in the JSON API by setting the value
to null; however this doesn't currently work through this library.
This PR allows users to clear metadata keys by setting the value
to None and documents how users should do this.

Fixes googleapis#381

google/cloud/storage/blob.py

@@ -3620,13 +3620,16 @@ def metadata(self):
     def metadata(self, value):
         """Update arbitrary/application specific metadata for the object.
 
+        Values are stored to GCS as strings. To delete a key, set its value to
+        None and call blob.patch().
+
         See https://cloud.google.com/storage/docs/json_api/v1/objects
 
         :type value: dict
         :param value: The blob metadata to set.
         """
         if value is not None:
-            value = {k: str(v) for k, v in value.items()}
+            value = {k: str(v) if v is not None else None for k, v in value.items()}
         self._patch_property("metadata", value)
 
     @property

tests/system/test_system.py

@@ -874,6 +874,19 @@ def test_write_metadata(self):
         blob.content_type = "image/png"
         self.assertEqual(blob.content_type, "image/png")
 
+        metadata = {"foo": "Foo", "bar": "Bar"}
+        blob.metadata = metadata
+        blob.patch()
+        blob.reload()
+        self.assertEqual(blob.metadata, metadata)
+
+        # Ensure that metadata keys can be deleted by setting equal to None.
+        new_metadata = {"foo": "Foo", "bar": None}
+        blob.metadata = new_metadata
+        blob.patch()
+        blob.reload()
+        self.assertEqual(blob.metadata, {"foo": "Foo"})
+
     def test_direct_write_and_read_into_file(self):
         blob = self.bucket.blob("MyBuffer")
         file_contents = b"Hello World"