Fix internal node weirdness

* In getDataElementOrSlice(), we were slicing one too far
* When encountering a closing for an internal node, don't traverse
  up. Doing this caused weird bugs, like inserting text where it didn't
  belong in some cases an exceptions in others, but these issues were
  parly masked by the off-by-one error in the data slice.

Change-Id: Ieda9afa95b7c1953d09e391774350a9b4148c2fe