Fix bug when processing blank nodes with reverse properties

This fixes a bug when creating a node map for blank nodes with reverse properties. See http://lists.w3.org/Archives/Public/public-linked-json/2013Oct/0046.html for the original bug report.

I've also added two new tests (`toRdf-0118` and `flatten-0044`) which test the fix and the EARL report for my implementation. None of the existing tests has been invalidated.