Encoding error when using json_pure adapter

Error appears from version 1.4.0 (it was working fine on 1.3.7).

```ruby
MultiJson.adapter = :json_pure
MultiJson.decode("{\"color\":\"żółć\"}")
MultiJson::DecodeError Exception: Caught Encoding::CompatibilityError at '{"color":"żółć"}': incompatible encoding regexp match (ASCII-8BIT regexp with UTF-8 string)
```

It works after forcing encoding to ASCII-8BIT:

```ruby
MultiJson.adapter = :json_pure
MultiJson.decode("{\"color\":\"żółć\"}".force_encoding("ASCII-8BIT"))
 => {"color"=>"żółć"}
```