fix 5.6 binary utf8 encoding, harmonized with newer perls.
5.6. encodes strings to utf8 internally when seeing a codepoint >= 0x80.
with binary decode it to the original bytes before encoding it to escaped
JSON.

Also fix the documentation for new binary multi-byte utf8 encodings:
* fail for 5.6,
* silently encode the utf8 representation with newer perls. (discussable)