Bug 20991: Do not lost patron's category when editing a patron

Bug 20991: Do not lost patron's category when editing a patron

If there is an error in the edit patron form the patron's category is
lost.
This seems to be a long standing bug.

Test plan:
- Edit an existing patron
- Change the patron category to a category that triggers the error that
the user is not in the right age range for that new category
- Save, error is triggered
=> Without this patch the patron category has been reset

You should also test different ways to edit/add a patron (quick add,
step 1)

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>