Patches bug where a pull request is able to be gifted multiple times.

To see bug:

1. Goto [new gift page](http://24pullrequests.com/gifts/new).
2. Without touching either of the select fields, click "Gift it!"
3. Page is rendered again, but now includes all of your pull requests (including already gifted ones).

The problem lies in the gifts controller, where it is inputting all of the pull requests into the form, and not just the `unspent_pull_requests`.