Consolidating duplicate code

SimpleDatastream and QualifiedDublinCoreDatastream had several duplicate
lines of code. And in some cases the behavior of SimpleDatastream appeared
to have patches that weren't applied to QualifiedDublinCoreDatastream.

There are some changes which alter the functionality of the base
NokogiriDatastream; But I've moved the shared behavior of both the 
SimpleDatastream and the QualifiedDublinCoreDatastream classes into their
parent NokogiriDatastream.

HYDRA-898