Make public constructors in CustomGzipOutputStream

[magicDGS]

Description

I made public the CustomGzipOutputStream in #746 to use it outside IOUtils, but I forgot to made the constructors public too. This fix the problem and makes the custom gzip wrapper available for use it outside the package.

Checklist

• Code compiles correctly
• New tests covering changes and new functionality
• All tests passing
• Extended the README / documentation, if necessary
• Is not backward compatible (breaks binary or source compatibility)

[magicDGS]

You accepted my previous PR, @lbergelson... could you have a look to this one?

[lbergelson]

@magicDGS 👍

[magicDGS]

Thanks!