Bug 28160: Values from 150$a aren't prefilled in z39.50 search form from an existing...

Bug 28160: Values from 150$a aren't prefilled in z39.50 search form from an existing authority record

For names and uniform titles, when you do a z39.50 search from an authority
record, we prefill what's in the relevant tag's subfield a in the search
form, but for subjects, we don't, both because GetMandatoryFieldZ3950 thinks
the param is topic when it's actually subject, and because
z3950_auth_search.tt doesn't bother putting the value of &subject into the
form.

Test plan:

1) Authorities - New authority - Topical Term
2) Tab 1, click on HEADING-TOPICAL TERM to expand subfields
3) In subfield a, type goats
4) Click Z39.50/SRU search and verify that the Subject heading search field
   has goats prefilled

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>