string: Add test for FromIterator<&str> and Extend<&str>

Author: bluss

src/libcollections/string.rs

@@ -1337,11 +1337,18 @@ mod tests {
         let u = "Việt Nam";
 
         let a: String = s.chars().collect();
-        assert_eq!(s, a.as_slice());
+        assert_eq!(s, a);
 
         let mut b = t.to_string();
         b.extend(u.chars());
-        assert_eq!(s, b.as_slice());
+        assert_eq!(s, b);
+
+        let c: String = vec![t, u].into_iter().collect();
+        assert_eq!(s, c);
+
+        let mut d = t.to_string();
+        d.extend(vec![u].into_iter());
+        assert_eq!(s, d);
     }
 
     #[bench]