Cast other types to JSON when compared with JSON value

[YangKeao]

Enhancement

In MySQL 5.7:

1149   if (type != ROW_RESULT &&
1150       (((*a)->result_type() == STRING_RESULT &&
1151         (*a)->field_type() == MYSQL_TYPE_JSON) ||
1152        ((*b)->result_type() == STRING_RESULT &&                                                                                                                                     
1153         (*b)->field_type() == MYSQL_TYPE_JSON)))
1154   {
1155     // Use the JSON comparator if at least one of the arguments is JSON.
1156     is_nulls_eq= is_owner_equal_func();
1157     func= &Arg_comparator::compare_json;
1158     return 0;
1159   }

If one of the compare args is json value, the other should be casted to json.