Small correctness tweak, probably in unused code.

* subversion/libsvn_ra_serf/eagain_bucket.c
  (eagain_bucket_peek): Report EAGAIN instead of EOF from the peek handler.

Suggested by: ivan


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557721 13f79535-47bb-0310-9956-ffa450edef68