UPDATE 1-Kenyan shilling rallies to 2013 peak ahead of elections

* Shilling firms on election optimism * Mumias Sugar shares tumble after profit warning By Kevin Mwanza NAIROBI, Feb 28 (Reuters) - The Kenyan shilling rallied on Thursday for a sixth session to trade at its strongest level so far this year, four days ahead of presidential and legislative elections. At 0908 GMT, Thomson Reuters data showed the shilling firmed 0.8 percent during the session to strike an intraday high of 85.60 against the dollar, last reached on Dec. 27, 2012.