Fixed Encoding::CompatibilityError under certain condition.
Fixed Encoding::CompatibilityError under certain condition.

`MessagePack.pack(obj, out)` or `obj.to_msgpack(out)` method raised
'incompatible character encodings: ASCII-8BIT and UTF-8' error if `out`
is a string and `obj` includes a string with non-ASCII-8BIT encoding and
longer than write_reference_threshold (5KB by defualt).

cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby,larskanis/msgpack-ruby,nurse/msgpack-ruby,kou/msgpack-ruby,ojundt/msgpack-ruby,msgpack/msgpack-ruby,larskanis/msgpack-ruby,okkez/msgpack-ruby,nurse/msgpack-ruby,msgpack/msgpack-ruby,ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,okkez/msgpack-ruby,msgpack/msgpack-ruby,ojundt/msgpack-ruby,nurse/msgpack-ruby,kou/msgpack-ruby,kou/msgpack-ruby,okkez/msgpack-ruby,larskanis/msgpack-ruby,larskanis/msgpack-ruby,kou/msgpack-ruby,nurse/msgpack-ruby,okkez/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby