Fix for reading chunked HTTP responses

`on_body` was appending more than it was supposed to when the callback was fired, this was very apparent in a chunked response.

This can easily be tested with the following which gets a webpage that is chunked up line by line:
```bash
julia -e 'using Requests; println(get("http://www.httpwatch.com/httpgallery/chunked/").data)'
```