git-svn: fix auth parameter handling on SVN 1.9.0+

git-svn: fix auth parameter handling on SVN 1.9.0+

For users with "store-passwords = no" set in the "[auth]" section of
their ~/.subversion/config, SVN 1.9.0+ would fail with the
following message when attempting to call svn_auth_set_parameter:

  Value is not a string (or undef) at Git/SVN/Ra.pm

Ironically, this breakage was caused by r1553823 in subversion:

  "Make svn_auth_set_parameter() usable from Perl bindings."

Since 2007 (602015e0e6ec), git-svn has used a workaround to make
svn_auth_set_parameter usable internally.  However this workaround
breaks under SVN 1.9+, which deals properly with the type mapping
and fails to recognize our workaround.

For pre-1.9.0 SVN, we continue to use the existing workaround for
the lack of proper type mapping in the bindings.

Tested under subversion 1.6.17 and 1.9.3.

I've also verified r1553823 was not backported to SVN 1.8.x:

  BRANCH=http://svn.apache.org/repos/asf/subversion/branches/1.8.x
  svn log -v $BRANCH/subversion/bindings/swig/core.i

ref: https://bugs.debian.org/797705
Cc: 797705@bugs.debian.org
Reported-by: Thierry Vignaud <thierry.vignaud@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Tested-by: Thierry Vignaud <thierry.vignaud@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>