Bug 30372: Fix value->attribute

Bug 30372: Fix value->attribute

Test plan:
- Activate patron self registration without email verification
- Create several patron attributes as visible and editable in the OPAC, make one mandatory
- Register as a new patron from the OPAC
- Fill in all required fields but the extended attribute
- Fill in at least one of the non-required extended attributes
- Submit
- Verify that the contents of the other extended attribute fields are still present.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>