Fix byte length counting if strlen overloading used

Function strlen() counts characters instead of bytes if mbstring.func_overload is used in php.ini so Content-Length header gets a wrong value.

I changed strlen() call to mb_strlen() with "8bit" encoding as suggested here http://www.php.net/manual/ru/function.mb-strlen.php#77040

May be we can even get rid of setting Content-Length explicitly?