Make saveDocument and createDocument return their passed DocumentModel

Currently developers have to write:

doc = session.createDocument(doc);

or

doc = session.saveDocument(doc);

because the input DocumentModel is not modified in place, and another DocumentModel is instantiated and returned. This is a pain and very error-prone.

We want to be able to write just:

session.createDocument(doc);

or

session.saveDocument(doc);

so that the input DocumentModel parameter is the one modified and returned to the caller.