Allow 'repository.create' to set description and autoclose

Summary:
These are currently not available via Conduit.

Also fix a bug where bad JSON input triggers an error about undefined $metadata.

Test Plan: Ran 'repository.create' with and without a description and with and without autoclose. Verified the created repositories had the requested attributes.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2881