Added test for trait From<T> for Ratio<T> where T: Clone + Integer.

rational/src/lib.rs

@@ -745,6 +745,7 @@ mod test {
         assert_eq!(_1_2, Ratio::new(1, 2));
         assert_eq!(_3_2, Ratio::new(3, 2));
         assert_eq!(_NEG1_2, Ratio::new(-1, 2));
+        assert_eq!(_2, From::from(2));
     }
 
     #[test]