Error tagging with ra_serf

In an effort to make sure my problems with serf are well known, I'm
posting this. I'm attempting to create a tag of a personal project,
and get the following error (over https, with the 1.7.x branch):

[[[
$ svn cp $REPOS/hikehy/trunk $REPOS/hikehy/releases/4.0.0
subversion/svn/copy-cmd.c:134: (apr_err=200007)
subversion/libsvn_client/copy.c:2346: (apr_err=200007)
subversion/libsvn_client/copy.c:1026: (apr_err=200007)
subversion/libsvn_client/copy.c:1026: (apr_err=200007)
subversion/libsvn_ra_serf/serf.c:614: (apr_err=200007)
subversion/libsvn_ra_serf/util.c:711: (apr_err=200007)
svn: E200007: Error running context
subversion/libsvn_ra_serf/util.c:711: (apr_err=200007)
svn: E200007: Trying to use an unsupported feature
$
]]]

After switching to neon, the commit went through just fine. The
server is running 1.6.17.

-Hyrum

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