Properly clone the document for the sanity check

Previously, we'd clone the data but convert it in the context of
the existing dm.Document, whose nodes had pointers to elements in the
old data array, not to the cloned ones. Because dm.MWReferenceNode
has logic like if ( something === dataElement ), this caused the sanity
check conversion to behave slightly differently compared to the real
conversion that happens on save, and so a references corruption
bug went unnoticed.

Change-Id: I79a42ae21f91cb8eb410ae26ea638036db19e217
Merge "Properly clone the document for the sanity check"