temprorarily fix for ping method

Compatible with the behavior of official MySQLdb, except that MySQLdb would raise ProgrammingError when autocommit == False.
query("select 1;") may change to query("show status;") whatever, just ugly test for the connection (is_connected() not works when kicked out by MySQL)
Merge pull request #2 from V-E-O/patch-1

temprorarily fix for ping method