Round trip (load-save) causes a 500 error.

[jcoyne]

This is caused by the default prefer header being: Prefer: return=representation

     Ldp::SaveException:
       STATUS: 500 org.fcrepo.kernel.exception.MalformedRdfException: Could not persist triple containing type assertion:http://www.jcp.org/jcr/nt/1.0hierarchyNode because no such mixin/type can be added to this node: /8e/1c/86/45!
        at org.fcrepo.kernel.utils.iterators.RdfAdder.operateOnMixin(RdfAdder.java:72)
        at org.fcrepo.kernel.utils.iterators.PersistingRdfStreamConsumer.operateOnTriple(PersistingRdfStreamConsumer.java:133)

A potential solution is to have the default prefer heading be Prefer: return=minimal