Only add our option headers in the mod_dav_svn OPTIONS handler if the client
asks for our option(s). This avoids setting them over and over again if a
client asks for a list of options at once.

* subversion/mod_dav_svn/version.c
  (get_option): Following up on r1557094, only add our option headers when
    asked for our option; not for every (undefined) option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1557103 13f79535-47bb-0310-9956-ffa450edef68