Update handler to merge consecutive 'cdata' instructions (incl. test case updates)

This patch is the second in the series designed to make it easier for DW to allow
plugins to modify the standard handling of line-breaks.

Like the first this patch doesn't alter line-break behaviour at all, but introduces
improvements that reduce to a minimum the number of 'cdata' instructions generated
by the handler.

darcs-hash:20080312000248-f07c6-f6ce1b5aac43a52cbe31215c517b048679ae20a7.gz