[Javascript] Fix handling of functions which take void*

Fixes #682

Examples/test-suite/voidtest.i

@@ -9,6 +9,7 @@ class Foo {
 public:
    Foo(void) { }
    void memberfunc(void) { }
+   void* get_this() { return this; }
    static void staticmemberfunc(void) { }
 };
 
@@ -18,4 +19,6 @@ void *vfunc2(Foo *f) { return f; }
 Foo  *vfunc3(void *f) { return (Foo *) f; }
 Foo  *vfunc4(Foo *f) { return f; }
 
+bool test_pointers_equal(void *a, void *b) { return a == b; }
+
 %}