Custom field checkboxes migrated incorrectly on merge

To recreate:

• Create a new custom field, type 'checkbox', two options.
• Create two records, one with the checkbox checked, one without.
• Use the manual merge screen to merge the two contacts
• Choose to take across the blank value from the 'other' to 'main' record.
• Note that the main record still has the checkbox ticked.

• As above, but give the 'other' record 'Option 2' and the main record 'Option 1'
• Merge, choosing to put 'Option 2' on to the main record
• Note that the main record still has 'Option 1'

'Adding' a value when it is blank on the 'main' record does work. Ie: An option is selected on the left, but nothing on the right. When migrated the main record does have the value.

Tested back to version 4.7.8