Add a test that unique boxes in vectors are copied when the vector is

Issue #409

src/test/run-pass/unique-in-vec-copy.rs

@@ -0,0 +1,13 @@
+fn main() {
+    let a = [~mutable 10];
+    let b = a;
+
+    assert *a[0] == 10;
+    assert *b[0] == 10;
+
+    // This should only modify the value in a, not b
+    *a[0] = 20;
+
+    assert *a[0] == 20;
+    assert *b[0] == 10;
+}