JDK-8242959 residual cleanup: Move encoding checks into ZipCoder

[Eirik Bjørsnøs]

> The following is purely a code cleanup/refactoring proposal for ZipFile
> internals:
>


> Here's a patch which implements the proposal. Sponsors are welcome :-)
>

I commited the proposal to a branch on my fork:
https://github.com/eirbjo/jdk/commit/be26f48f94b5eb8e9566a66e0509be998f34977f

Give me a hint if I should create a PR for this (not totally up to speed
with the current Skara process for such small changes)

Cheers,
Eirik.