Whitespace preservation was broken after the first run

The first run of getDomFromData() would preserve whitespace just fine,
but it blanked out the .veInternal.whitespace[1] element in certain
cases, contaminating the linear model and making the whitespace data
inconsistent. Subsequent runs of getDomFromData() would then refuse to
serialize that whitespace because the information about it was
inconsistent.

In getDomFromData(), we sometimes unset .veInternal.whitespace[1] (i.e.
set it to undefined) to prevent double processing. Because we're
potentially going to modify .veInternal, don't assign it by reference,
but copy the object.

Added tests asserting that the linear model is unchanged after calling
getDomFromData(), because that function should never modify
linear model data. This test failed in 4 cases (all whitespace-related)
before I added the copyObject() call.

Bug: 43543
Change-Id: Ic4c93510518163894201a693ab50331413715967