Bug 18246: Patron card creator: Unit not always displays properly in layouts

Bug 18246: Patron card creator: Unit not always displays properly in layouts

To reproduce:

1) Create a new layout
2) Select an unit in "Units selection"
3) Save
4) Edit layout again
5) Select other unit, e.g. 'SI Centimeter'
6) Save
7) Repeat
8) Edit again
Error: Latest unit seems to stick and can no longer be changed, because
in units drop down, multiple untis are marked as selected.

To test:
1) Apply patch
2) Repeat steps above, verify that value for units always display as expected.

Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>