Export jcr/xml without binary contents by setting the parameter skitBinary

https://www.pivotaltracker.com/s/projects/684825/stories/74701156

When append the parameter skipBinary=true to the original fcr:export url like http://localhost:8080/rest/file/fcr:export?skipBinary=true, we can get the jcr/xml output with no binary contents now.  
I am manipulate to avoid adding another signature like serialize(final FedoraObject obj, final OutputStream out, boolean skipBinary) to the FedoraObjectSerializer interface for this special output at this time. But I think we can if this should be a more general support for other future formats that Fadora supports. 
Change comment to:
// Contains the child node otherwise