allow nested functions as second parameter to DATE_* functions

Using sqlite as backend you can't pass nested function for the second parameter of DATE_* functions since it is transformed to a string. I've fixed that concatenating the string expression passed to sqlite DATE and DATETIME function using the getConcatExpression method - does this seem correct?

The DATE_* functions work fine with nested second parameter in mysql.

I've added assertions similar to other platforms for the date functions.
What about other platforms?