Delegate the iprop ra command to the ra layers even when they don't report
that they support iprops to allow them to use a more efficient implementation
than calling the normal property obtain code in a loop.

This in preparation of adding an optimized implementation in ra_serf.
(I can't really think of a way to optimize this in ra_svn)

* subversion/libsvn_ra/ra_loader.c
  (svn_ra_get_inherited_props): Always call the function in the vtable and
    only fallback to default implementation when it returns the magic error.

* subversion/libsvn_ra_serf/inherited_props.c
  (svn_ra_serf__get_inherited_props): Check how to obtain iprops.

* subversion/libsvn_ra_svn/client.c
  (ra_svn_get_inherited_props): Check how to obtain iprops.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1552455 13f79535-47bb-0310-9956-ffa450edef68