add a send overload

[ringabout]

ref dom96/jester#241

adding a way supporting user-defined Content-Length

usage:

  if headers.hasKey("Content-Length"):
    request.send(code, body, some(headers["Content-Length", 0]), headers.createHeaders)
  else:
    request.send(code, body, headers.createHeaders)

[dom96]

At this point, can't the framework just use unsafeSend?

[ringabout]

I see.