JavaScript: Stop breaking surrogate pairs in toDelta()

Resolves google#69 for JavaScript

Sometimes we can find a common prefix that runs into the middle of a
surrogate pair and we split that pair when building our diff groups.

This is fine as long as we are operating on UTF-16 code units. It
becomes problematic when we start trying to treat those substrings as
valid Unicode (or UTF-8) sequences.

When we pass these split groups into `toDelta()` we do just that and the
library crashes. In this patch we're post-processing the diff groups
before encoding them to make sure that we un-split the surrogate pairs.

The post-processed diffs should produce the same output when applying
the diffs. The diff string itself will be different but should change
that much - only by a single character at surrogate boundaries.