Bug 29059: Keep non-repeatable attribute from patron to preserve when merging

Bug 29059: Keep non-repeatable attribute from patron to preserve when merging

See bug 21648 comment 17.

Suggestion is to keep the non-repeatable patron's attribute from the patron we selected instead of raising a blocking error.

A side-effect will be that when several patrons are merged, the
non-repeatable attribute from the first one will be kept, which can
result in unexpected result if the original patron does not have the
attribute defined.

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>