New option: forceResponseCharset

# forceResponseCharset
This option allows user to force the charset of the response. This is useful when no charset is specified in response.

# Example usage
```
rest.post('http://twaud.io/api/v1/users/danwrong.json', {
  forceResponseCharset: 'iso-8859-15'
}).on('complete', function(data) {
  console.log(data[0].message); // auto convert to object
});
```