When setting a file as binary using the DOM, writing it out would fail,

due to the wrong parameter being passed to DiffXWriter.write_diff().

This needed to be diff_type=, but it was being passed as type=. We

now use the option remapping support in DiffXDOMWriter to pass the

correct argument.
Statistics were also being generated for binary diffs, which didn't make

any sense. We now skip generating stats for these.

Unit tests pass on Python 2 and 3.
Verified this fix in production.