Fix expansion of custom keywords with values that contain '='.

* subversion/libsvn_subr/subst.c
  (build_keywords): Don't use svn_cstring_split() to find the '=' separator.
    It is not a suitable API for this purpose. Instead, scan for the first
    occurrence of '=' and split keyword name from custom format there.

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_svn_subst_build_keywords3): Expand to cover a custom keyword with '='.
   Also assert that a non-NULL expanded keyword value is returned to avoid
   crashing this test with a NULL-deref.

Reported by: Lorenz (lorenznl -@T- yahoo.com)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486072 13f79535-47bb-0310-9956-ffa450edef68