pass-through the implementation of is_zero instead of comparing to ze…

…ro manually.

also test is_zero.

src/system.rs

@@ -820,7 +820,7 @@ macro_rules! system {
             }
 
             fn is_zero(&self) -> bool {
-                self.value == V::zero()
+                self.value.is_zero()
             }
         }
 

src/tests.rs

@@ -391,6 +391,7 @@ mod system_macro {
         fn zero() {
             let z = Length::zero();
             assert_eq!(z.value, 0.0);
+            assert!(z.is_zero())
         }
 
         quickcheck! {