Bug 20581: Provide status_alias on IllRequest

Bug 20581: Provide status_alias on IllRequest

This patch provides status_alias support for the Koha::Illrequest
object.

- New 'statusalias' getter method, returns the request's status_alias
authorised value object if set, otherwise undef
- Overloads existing 'status' method, getter/setter accessors intact,
but setting the request's status now implicitly removes any status_alias
that has been set

Signed-off-by: Niamh.Walker-Headon@it-tallaght.ie
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>