Correctly sets the date on a new gift when one is provided.

When creating a code gift from the calendar screen, the date select is not pre-filled. See screenshot:

![24 pull requests 2013-12-01 22-34-00](https://f.cloud.github.com/assets/298309/1652139/0414c0b6-5b03-11e3-8682-ea99974d8f13.jpg)

This PR fixes the bug and adds a test to the gift controller spec to cover it.