fixing ZipPackagePart.GetStreamCore crashes with NotSupportedException (dotnet/corefx...

fixing ZipPackagePart.GetStreamCore crashes with NotSupportedException (dotnet/corefx#40319)

* fixing ZipPackagePart.GetStreamCore crashes with NotSupportedException

ZipArchiveEntry only ever supports opening once when the backing archive is in Create mode,  and the backing stream is non-seekable, so we shouldn't call SetLength in that case. You could still open an archive in Update mode then call part.GetStream(FileMode.Create), in which case we'll want this call to SetLength, so we only avoid this call when the backing Archive is in Create mode.

* updating test to explicitly test the Update path for ZipPackage

* skip UAP since we don't have access to the file system to create the .zip

* undo accidental change to existing test

* removing unnecessary variable

Commit migrated from https://github.com/dotnet/corefx/commit/8ac53e0c022b7881f659e0838a25e91f2ec5c8c0