Revert r1557721, as returning APR_EGAIN is invalid. Returning SUCCESS but NULL
data is not clean either, so just return APR_EOF as before.

* subversion/libsvn_ra_serf/eagain_bucket.c
  (eagain_bucket_peek): Revert behavior.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557725 13f79535-47bb-0310-9956-ffa450edef68