STR_TO_DATE() function if there is another format specifier after %f 
in format string". Also small cleanup of STR_TO_DATE() implementation.
(After review version.)


mysql-test/r/date_formats.result:
  Added test for small bug in STR_TO_DATE() implementation which caused
  microseconds to be gobbled from string result of this function, if
  there was another specifier after %f in format string.
mysql-test/t/date_formats.test:
  Added test for small bug in STR_TO_DATE() implementation which caused
  microseconds to be gobbled from string result of this function, if
  there was another specifier after %f in format string.
sql/item_timefunc.cc:
  Small cleanup of str_to_date() implementation.
    Renamed check_result_type() to less ambigous get_date_time_result_type()
    and made it static. Also added handling of %X,%x,%V,%v to this function.
    Fixed small bug in it which caused microseconds to be gobbled if there
    was some other specifiers after %f.
    Cleaned up comments a bit.