Maintain dietary state when form is invalid.

Previously we only set the dietary restrictions if the RSVP was saved.
Now we set them before attempting to save the RSVP. (Rails will save the
dietary restriction changes only if the rest of the RSVP is saved.)

Fixes #256.
Merge branch 'maintain-dietary-state-when-form-is-invalid' of https://github.com/eostrom/bridge_troll into eostrom-maintain-dietary-state-when-form-is-invalid