Slight modification of the policy about xlink:href. By default, write XLink attributes even if the metadata object is not nil.
The XLink is then for informative purpose. This change is made on the assumption that this behavior will be less surprising
for the users than omission of the metadata object from the XML output.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK7@1550306 13f79535-47bb-0310-9956-ffa450edef68