Make sure data-parsoid is set on document.body

* Eliminates spurious syntactic diffs in roundtrip testing because
  of one trailing newline separator emitted on EOF in some cases.

* Test case: node roundtrip-test.js Jehan_de_Bethancourt
  - Eliminates syntactic diffs on this and other pages.

Change-Id: Ieb59e18c892d6372970b9d4ce61bf77df5c6846f