Fix for #398

Change AlterTableExpressionBuilder to alter default values on existing
columns correctly.

The simplest way of doing this was to pass through from WithDefault to WithDefaultValue as that does exactly what I wanted. I also did the same thing in AlterColumnExpressionBuilder for consistency as the two method bodies were identical.

Tested with the two examples given by @derekgreer on #398. Before this change the migration claimed to succeed but no change was made to the database. With this change the migration changes the database as expected.