In ra_svn's marshaller, provide a dedicated function to send booleans over
the wire.  Call it where appropriate.

* subversion/include/private/svn_ra_svn_private.h
  (svn_ra_svn__write_boolean): declare new private API

* subversion/libsvn_ra_svn/marshal.c
  (svn_ra_svn__write_boolean): implement new private API
  (vwrite_tuple_boolean,
   write_tuple_boolean): call the new function instead of copy'n'pasting code

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486011 13f79535-47bb-0310-9956-ffa450edef68