Smart-quote serialized HTML for more compact readable output.

Note that we use standard serialized HTML (with double-quoted attributes)
when doing normalization for running tests.  The serializeNode()
output is only used when sending data to the user; it makes it much
easier to read data-parsoid attributes!

Change-Id: I0d86efcfc927a2dea71964d7ed26dc486efc0033
Merge "Smart-quote serialized HTML for more compact readable output."