newWebhook POST method missing git_ref_pattern argument

[Giuseppe Barbieri]

Recently webhooks introduced the git_ref_pattern field, a pattern to match against git-ref/branch name of an event, to filter webhook triggers.
The field is available in the webhook object and marked as writable, however it appears to not be exposed in the addWebhook method available for git_repositories.
When calling:
repo.newWebhook(delivery_url="https://my_url", event_types=["git:push:0.1", "merge-proposal:0.1"], git_ref_pattern="*foo*")

it returns:

ValueError: Unrecognized parameter(s): 'git_ref_pattern'

Should this field be made optionally available?