Fix the problem with chunked encoding with an empty body.

When the response body is empty, rocket won't send the final zero-length
terminating chunk for chunked encoding. I think this causes the browser
to wait for the connection close in order to tell the end of the
response.
Avoid sending the terminating chunk in case it's a HEAD request.
Merge pull request #567 from kjkuan/fix-chunked-encoding-with-empty-body

Fix the problem with chunked encoding with an empty body.