fix newline handling logic

Hi,
Newlines in stream must update @last_response.timestamp.
https://dev.twitter.com/docs/streaming-apis/messages#Blank_lines

This commit https://github.com/tweetstream/em-twitter/commit/470d7be82ca421118384397b36a6ff1c0c43f4e7 may invoke no_data_received in slow streams.