Make svn_auth_set_parameter() usable from Perl bindings.

* subversion/bindings/swig/core.i
  The third parameter (const void *value) to svn_auth_set_parameter() didn't
  have a typemap(in) for Perl. Follow the Python and Ruby bindings: 
  accept a scalar (will be passed as a C string) and undef (will be passed
  as NULL, i.e. deletes the auth_baton parameter).


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1553823 13f79535-47bb-0310-9956-ffa450edef68