Do not response chunked body to HTTP/1.0 client

rack-streaming-proxy broke the response body on the following condition.
 
* client request with HTTP/1.0 and the origin server responses with `Transfer-Encoding: chunked` with HTTP/1.1, 
* And rack-streaming-proxy send a chunked body to the client.

This was not compliant with RFC2145. A proxy must not sent chunked body to HTTP/1.0 client.
 
cf. https://gist.github.com/niku4i/9067161