ISO-8601 strings with no/missing units don't return an error

[Bhupesh-V]

At the moment, the Parse method successfully parses the following invalid date strings. Returns a zero Duration.

1. P6: No unit after prefix P.
2. P7Y4: No unit after valid sub-prefix.

Would you like a fix for this? Thoughts?

[ccoVeille]

I feel like you should.

[sosodev]

@Bhupesh-V Sure, a fix would be great :)

[rusq]

I went ahead and created a PR for this (#43).