BZ4376 Minor change in GetMarcAuthor

BZ4376 Minor change in GetMarcAuthor

A minor change in the GetMarcAuthors function of C4/Biblio.pm allow differentiate the type of authors in the templates

This change allow doing things like this in the templates:
<TMPL_IF EXPR="tag == 700" && code eq 'a' >
 <strong>Author:</strong>
<!-- TMPL_ELSE -->
 <TMPL_IF EXPR="tag == 710" && code eq 'a' >
  <strong>Corpotation Author:</strong>
<!-- /TMPL_IF -->
<!-- /TMPL_IF -->

(html template syntax, but also applicable to template toolkit)

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Works as claimed and doesn't break existing functionality.