fix #299 roundtrip errors on TriX and various other syntaxes

The problem comes from the fact that these syntaxes have explicit blank
node ids outputted, where Turtle, for example, outputs [] nodes
repeating triples when structure sharing happens.
In TriX, duplicate triples are not emitted and shared structure is
outputted only once. On parsing, though, the triples get consumed the
first time they are found (which is expected behaviour, see reference),
and the second reference to the blank node does not have any triples
associated, hence the Error1 situation.

http://www.w3.org/TR/owl-parsing/#subsec-structure-sharing