Merge the r1514315 group from trunk:

 * r1514315, r1514628
   Use loaded serf version for User-Agent, not version used for compilation.
   Justification:
     Wrong User-Agent string is not good thing.
   Votes:
     +1: ivan, danielsh, philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1514946 13f79535-47bb-0310-9956-ffa450edef68