Add a test for mutable references to unique boxes as function arguments

Issue #409

src/test/run-pass/unique-fn-arg-mut.rs

@@ -0,0 +1,9 @@
+fn f(&i: ~int) {
+    i = ~200;
+}
+
+fn main() {
+    let i = ~100;
+    f(i);
+    assert *i == 200;
+}