This prevents the acq unified_search method from returning anything other

This prevents the acq unified_search method from returning anything other
than the objects of the expected type.

This is to address LP 884991 in which Acq users get a perm error for each
result of a search that they don't have access to retrieve.  They shouldn't
even know about the existence of anything they don't have access to retrieve.

Signed-off-by: James Fournie <jfournie@sitka.bclibraries.ca>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>