Fixed 0003795: Resolve should not overwrite handler, if already set.

M core/bug_api.php

• (bug_resolved): Changed to only set the handler_id if was not set before, or set explicitly by the caller rather than parameter was defaulted.

git-svn-id: http://mantisbt.svn.sourceforge.net/svnroot/mantisbt/trunk@2503 <a class="text" href="/?p=mantisbt.git;a=object;h=f5dc347c">f5dc347c</a>-c33d-0410-90a0-b07cc1902cb9