When a node is dumped with a new encoding, we may encounter characters
that are not supported in the new encoding. libxml2 handles this by
replacing the character with character references, but in some encodings
this can result in an infinite loop when the character references
themselves contain unsupported characters.

This fixes the infinite loop by undoing a character reference substitution
when it cannot be inserted, and returning an encoder error.

This bug was noticed when looking into an infinite loop bug report for
the Ruby Nokogiri project. The original bug report, "nokogiri process
hangs on call to inner_html" is here:
https://github.com/tenderlove/nokogiri/issues/400