Storage: str() metadata for bucket and blob

[frankyn]

Surfaced by GCS team.

The Cloud Storage library must stringify metadata sent in bucket and blob.

Here's a known issue from not doing so:

from google.cloud import storage
client = storage.Client(project="my-project")
my_bucket = client.get_bucket("my-bucket")
my_blob = my_bucket.get_blob("my-blob")
my_blob.metadata["bad"] = float('nan')
my_blob.patch()

myblob.patch() will send an unquoted NaN, which is a bug in the Cloud Storage API that the client should no longer depend on.