Corrected the patch that made the optimizer switch for index condition pushdown

set to 'on' by default.

sql/mysqld.cc

@@ -481,7 +481,7 @@ static const char *optimizer_switch_str="index_merge=on,index_merge_union=on,"
                                         "index_merge_sort_union=on,"
                                         "index_merge_intersection=on,"
                                         "index_merge_sort_intersection=off,"
-                                        "index_condition_pushdown=off,"
+                                        "index_condition_pushdown=on,"
                                         "derived_merge=off,"
                                         "derived_with_keys=off,"
                                         "firstmatch=on,"