Missing ':' delimiter when using "zero pad" flag

[Jens Reimann]

When using the zero pad flag ({cpe:0}) not only the value, but also the : is being omitted.

Which might lead to fields being shifted. For example:

    #[test]
    fn test_zero() {
        use cpe::cpe::Cpe;

        let original = "cpe:/a:foo:bar:1.2::x";
        let parsed = OwnedUri::from_str(original).unwrap();

        let formatted = format!("{parsed:0}");
        let reparsed = OwnedUri::from_str(&formatted).unwrap();

        assert_eq!(parsed.part(), reparsed.part());
        assert_eq!(parsed.vendor(), reparsed.vendor());
        assert_eq!(parsed.product(), reparsed.product());
        assert_eq!(parsed.version(), reparsed.version());
        assert_eq!(parsed.update(), reparsed.update());
        assert_eq!(parsed.edition(), reparsed.edition());
    }

The test will fail for:

assert_eq!(parsed.update(), reparsed.update());

With:

assertion `left == right` failed
  left: Any
 right: Value("x")

Left:  Any
Right: Value("x")

As the value shifted:

• Original: cpe:/a:foo:bar:1.2::x
• Formatted: cpe:/a:foo:bar:1.2:x