Add a separate finished flag to PipedRDFIterator which is set when the end marker is seen, if this is set hasNext() returns false.

This is necessary because setting closedByReader (as was previously done and I removed already) caused hasNext() to throw a spurious exception if the caller invoked hasNext() multiple times.  Setting a separate flag allows us to properly return false

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439864 13f79535-47bb-0310-9956-ffa450edef68