Fix issue where ChromePHP and FirePHP handlers sent logs to ineligible browsers when >1 handler is instantiated

I noticed that when I have multiple instances of either ChromePHPHandler or FirePHPHandler, that some messages are being delivered when the requesting browser's user agent should have prevented this.  It seems that the static $Initialized variable determines whether or not to check the requesting user agent, so only one handler does the check while any other instances of that handler default to sending the headers without doing a check.