Default zip ver to 20 (deflate/encyption), fixes #164

[damieng]

Sets the default level of '20' for zip files in the ZipCentralDirectoryBlock to match '20' used in Zip Local File Headers at https://github.com/adamhathcock/sharpcompress/blob/master/src/SharpCompress/Writers/Zip/ZipWriter.cs#L162

This makes Microsoft's compression.io library used by NuGet happy (and possibly some other tools)

[damieng]

Let me know if you want any test coverage.

[adamhathcock]

I guess ideally there should be a test validating against packaging. Not sure that's possible on netcoreapp. Probably need to make another test project on full framework.

I plan on doing a bit more work then a release Monday to get this and other fixes out.

[damieng]

Thanks for fast turnaround. We could just have a test that checked the fields are the same rather than testing with System.Packaging.

[adamhathcock]

Could do. Whatever is easier.

I may explore packinging myself. Integration feel the most valuable when trying to match external programs. Though, in this case, it's being consistent internally was the problem.

Not easy to test this stuff .