add (HTTP::Message::Body::Parts)

    # File lib/httpclient/http.rb, line 540
            def add(part)
              if Message.file?(part)
                @as_stream = true
                @body << part
                if part.respond_to?(:size)
                  if sz = part.size
                    @size += sz
                  else
                    @size = nil
                  end
                elsif part.respond_to?(:lstat)
                  @size += part.lstat.size
                else
                  # use chunked upload
                  @size = nil
                end
              elsif @body[-1].is_a?(String)
                @body[-1] += part.to_s
                @size += part.to_s.size if @size
              else
                @body << part.to_s
                @size += part.to_s.size if @size
              end
            end