Add test for #5239

I've added a test for the second example mentioned in #5239. The first example does not compile with a reasonable error message. Should I add a compile-fail test for that example as well?

    /rust/src/test/run-pass/issue-5239.rs:15:45: 15:51 error: binary operation + cannot be applied to type `&int`
    rust/src/test/run-pass/issue-5239.rs:15     let _f = |ref x: int| { x += 1};
                                                                                         ^~~~~~

    error: aborting due to previous error