Ensure previous vote choices are remembered.

[jholdstock]

This change ensures that old voting preferences are always remembered by the DB. The API for setting vote choices was previously a bit forgetful. Eg.

1. Client sets vote choices { a: yes, b: yes }
2. VSP stores { a: yes, b: yes }
3. Client sets vote choices { a: no, c: no }
4. VSP stores { a: no, c: no }

b: yes would previously have been forgotten, but now it is remembered