Add a test for assignment of unique boxes of generics

Closes #976

Issue #409

Author: brson

src/test/run-pass/unique-generic-assign.rs

@@ -0,0 +1,6 @@
+// Issue #976
+
+fn f<@T>(x: ~T) {
+    let _x2 = x;
+}
+fn main() { }