Expression null exception fix

When performing a method call in a Select statement, the
SqlExpressionVisitor checks if it is a Sql, Array or Column method call.
However a NullReferenceException is thrown when trying to do this:
db.Select(q=>q.DateStamp < DateTime.Now.AddMonths(-1));

Now the above call is properly expressed.
Remove additional casts
Merge pull request #149 from dylanvdmerwe/master

Expression NullReferenceException fix.