Add a test for unique boxes as fn args

Issue #409

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

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