Update CGridView.php

Added a seam to override only a data cell rendering process, without the need to override the whole `CGridView.renderTableRow()`.
Marked the change to CGridView.renderDataCell() from c8969e543bdf0075f895ff92d460b5a2af4e9aa1 as introduced in version 1.1.17
re #3571
Merge pull request #3571 from hijarian/patch-1

Update CGridView.php