Regression: When batch updating custom fields, allow multi-value fields to be added to or removed from rather than updated

Regression of CRM-3633

Steps to reproduce:
1) Create a custom multi-value field.
2) Add the field to a profile.
3) Edit a contact and set a few options for that field.
4) Run a search that returns this contact and a few others. The test contact should not be the first in the list.
5) Choose the search results action 'Batch update via profile'.
6) Choose the profile.
7) Add a few options for the mulit-value field (different from the set of options chosen in #3).
8) Click the copy icon to copy the values to all contacts.
9) Observe that the previous values for the test contact have been walloped.