Added example to show the AsRef working

exercises/conversions/as_ref_mut.rs

@@ -36,4 +36,16 @@ mod tests {
         let s = "Cafe au lait";
         assert_eq!(char_counter(s), byte_counter(s));
     }
+
+    #[test]
+    fn different_counts_using_string() {
+        let s = String::from("Café au lait");
+        assert_ne!(char_counter(s.clone()), byte_counter(s));
+    }
+
+    #[test]
+    fn same_counts_using_string() {
+        let s = String::from("Cafe au lait");
+        assert_eq!(char_counter(s.clone()), byte_counter(s));
+    }
 }