Use response character encoding

I think response character encoding should be used if this value was set or else default to "url-8" value.
@perwendel It shouldn't be null indeed, Jetty's Response class implements it as following
```java
    @Override
    public String getCharacterEncoding()
    {
        if (_characterEncoding == null)
            _characterEncoding = StringUtil.__ISO_8859_1;
        return _characterEncoding;
    }
```