Added support for dynamic columns to select_expression_list

Dynamic columns are not yet supported in results, though.

tests/SelectTest.cpp

@@ -25,9 +25,10 @@
 
 #include "TabSample.h"
 #include <sqlpp11/select.h>
+#include <sqlpp11/connection.h>
 
 #include <iostream>
-class DbMock
+class DbMock: public sqlpp::connection
 {
 public:
 	const std::string& escape(const std::string& text) { return text; }
@@ -273,6 +274,12 @@ int main()
 		std::cerr << "------------------------\n";
 	}
 
+	// Test that select can be called with zero columns if it is used with dynamic columns.
+	{
+		auto s = dynamic_select(db).dynamic_columns().add_column(t.alpha);
+		s.serialize(std::cerr, db);
+	}
+
 
 	static_assert(sqlpp::is_select_flag_t<decltype(sqlpp::all)>::value, "sqlpp::all has to be a select_flag");
   using T = sqlpp::detail::wrap_operand<int>::type;