Preserve data-mw verbatim if unchanged

Previously we would reserialize the JSON blob in data-mw even if we
didn't change it, which potentially reordered keys and caused a DOM
diff.

Bug: 50066
Change-Id: If0a5bcc67d3a172de0e8839cfda11efacfbf36ff
Merge "Preserve data-mw verbatim if unchanged"