added wxMBConv::Clone() to be able to copy conversion objects polymorphically

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@38576 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/convauto.h

@@ -40,6 +40,8 @@ public:
 
     virtual size_t GetMBNulLen() const { return m_conv->GetMBNulLen(); }
 
+    virtual wxMBConv *Clone() const { return new wxConvAuto(*this); }
+
 private:
     // all currently recognized BOM values
     enum BOMType