Fixes Issue #106: deep_merge recursively slow

The call to custom_writer within deep_merge was exponentially converting the values (unnecessarily, since they were just converted within deep_merge).  Added an optional param to custom_writer to suppress the conversion.  Unfortunately, had to also add the param to coercion's customer_writer method as well (which is just ignored) to not break the inheritance.

All tests pass.
Added spec for #107.
Added a 1-liner to CHANGELOG.
Merged with master.