I want to add a `predefined_query_handler` to serve a single JSON from my query.
For example

```xml
<rule>
      <url>/my/url</url>
      <methods>GET</methods>
      <handler>
        <type>predefined_query_handler</type>
        <query>select '{"my": "json"}' format JSONAsString</query>
      </handler>
    </rule>
```

I can use `RawBLOB` but it sets up Content-Type 'text/plain' instead of 'application/json'.
I think JSONAsString should also work with output.

*response.send() << s << std::endl;
    }
    else if (used_output.out_maybe_compressed)
    {
        
        if (used_output.hasDelayed())
            used_output.out_maybe_delayed_and_compressed.reset();
        
        
        
        
        bool data_sent = used_output.out->count() != used_output.out->offset();
        if (!data_sent)
        {
            used_output.out_maybe_compressed->position() = used_output.out_maybe_compressed->buffer().begin();
            used_output.out->position() = used_output.out->buffer().begin();
        }
        writeString(s, *used_output.out_maybe_compressed);
        writeChar('\n', *used_output.out_maybe_compressed);
        used_output.out_maybe_compressed->next();
    }
    else
    {
        assert(false);
        __builtin_unreachable();
    }
    used_output.finalize();
}
catch (...)
{