m: Fix new nightly warning (#1176)

Signed-off-by: John Nunley <dev@notgull.net>

tests/path/arg.rs

@@ -13,7 +13,7 @@ fn test_arg() {
     use std::borrow::Borrow;
 
     let t: &str = "hello";
-    assert_eq!("hello", t.as_str().unwrap());
+    assert_eq!("hello", Arg::as_str(&t).unwrap());
     assert_eq!("hello".to_owned(), Arg::to_string_lossy(&t));
     assert_eq!(cstr!("hello"), Borrow::borrow(&t.as_cow_c_str().unwrap()));
     assert_eq!(cstr!("hello"), Borrow::borrow(&t.into_c_str().unwrap()));