code: func (r *Request) ContentLength() int64 {
  	// TODO(dfc) this should support anything with a Len() int64 method.
  	if r.Body == nil {
  		return -1
  	}
  	switch b := r.Body.(type) {
  	case *bytes.Buffer:
  		return int64(b.Len())
  	case *strings.Reader:
  		return int64(b.Len())
  	default:
  		return -1
  	}
  }
docs: // ContentLength returns the length of the body. If the body length is not known
  // ContentLength will return -1.