getSqlExpression() should not be final

[dugwood]

If we need to extend the dialect, we have to overload the getSqlExpression(). See #2952 and http://forum.phalconphp.com/discussion/1748/date-sub-interval-mysql

[dugwood]

My build is SegFaulting since this change... So there must be an issue with all this. As soon as dialectClass is used, almost all queries are going in Segmentation fault:

#0  zend_call_function (fci=0x7fff4f0be280, fci_cache=0x8) at /tmp/buildd/php5-5.4.34/Zend/zend_execute_API.c:837
#1  0x00007f6e6bc7fb9a in zephir_call_user_function () from /usr/lib/php5/20100525/phalcon.so
#2  0x00007f6e6bc621a6 in zephir_call_class_method_aparams () from /usr/lib/php5/20100525/phalcon.so
#3  0x00007f6e6ba9ffc2 in zim_Phalcon_Db_Dialect_select () from /usr/lib/php5/20100525/phalcon.so
#4  0x000000000069ba6d in zend_call_function (fci=0x7fff4f0be8a0, fci_cache=0x19053e8) at /tmp/buildd/php5-5.4.34/Zend/zend_execute_API.c:978
#5  0x00007f6e6bc7fc8f in zephir_call_user_function () from /usr/lib/php5/20100525/phalcon.so
#6  0x00007f6e6bc621a6 in zephir_call_class_method_aparams () from /usr/lib/php5/20100525/phalcon.so
#7  0x00007f6e6ba367ba in zim_Phalcon_Mvc_Model_Query__executeSelect () from /usr/lib/php5/20100525/phalcon.so

[phalcon]

Thanks