Fix assertion handling defaultargs

Occurs when using %extend for a template class and the extended methods
contain default arguments.

Closes #611

Examples/test-suite/template_default_arg_overloaded_extend.i

@@ -0,0 +1,44 @@
+%module template_default_arg_overloaded_extend
+
+%inline %{
+namespace gaia2 {
+
+struct Filter {};
+struct SearchPoint {};
+struct DataSet {};
+
+template <typename SearchPointType, typename DataSetType>
+class BaseSearchSpace {};
+
+template <typename SearchPointType, typename DataSetType>
+class BaseResultSet {
+public:
+  const char *over(int i = 0) {
+    return "over(int)";
+  }
+};
+}
+%}
+
+// Specialized template extend
+%extend gaia2::BaseResultSet<gaia2::SearchPoint, gaia2::DataSet> {
+  int go_get(int n, gaia2::SearchPoint, int offset = -1) {
+    return offset;
+  }
+  const char *over(gaia2::SearchPoint, int x = 0) {
+    return "over(giai2::SearchPoint, int)";
+  }
+}
+
+// Generic extend for all template instantiations
+%extend gaia2::BaseResultSet {
+  int go_get_template(int n, SearchPointType sss, int offset = -2) {
+    return offset;
+  }
+  const char *over(bool b, SearchPointType, int x = 0) {
+    return "over(bool, SearchPointType, int)";
+  }
+}
+
+%template(ResultSet) gaia2::BaseResultSet<gaia2::SearchPoint, gaia2::DataSet>;
+