[Go] Fix member variables in base classes to handle CWRAP_NATURAL_VAR

correctly.  Add a test case for the problem.

Fixes #339.

Examples/test-suite/inherit_member.i

@@ -0,0 +1,17 @@
+// Based on https://github.com/swig/swig/issues/339 .
+
+%module inherit_member
+
+%include <std_string.i>
+
+%inline %{
+
+struct parent {
+  std::string pvar;
+};
+
+ struct child : public parent {
+  std::string cvar;
+};
+
+%}