add roundtrip tests for all i16/u16 (#545)

Author: alex

tests/roundtrip_tests.rs

@@ -21,6 +21,20 @@ fn test_i8() {
     }
 }
 
+#[test]
+fn test_u16() {
+    for i in u16::MIN..=u16::MAX {
+        assert_roundtrips::<u16>(i);
+    }
+}
+
+#[test]
+fn test_i16() {
+    for i in i16::MIN..=i16::MAX {
+        assert_roundtrips::<i16>(i);
+    }
+}
+
 #[test]
 fn test_u64() {
     for v in [0, 12356915591483590945, u64::MAX] {