Make it possible to add a ZipEntry from another zip file

[romerod]

This is the PR propessed in the #209.

Usage is like this:

using (var zip1 = ZipFile.Read(stream1))
using (var zip2 = ZipFile.Read(stream2))
{
    foreach (var entry in zip2.ToArray())
    {
        var newEntry = entry.CloneForNewZipFile(zip1);        
        newEntry.FileName = $@"zip2/{entry.FileName}";
        zipMain.AddEntry(newEntry);
    }
    zip1.Save(@"C:\temp\merged.zip");
}

[darkms]

Could you please add a check like this one for _Encryption and _Password?

[romerod]

I had the check on Encryption and Password, but had to remove them as they are not set on the zip1.

As I checked encryption can be mixed and as I don't decrypt I don't need the password.

Thanks for the fast review!

[darkms]

As I checked encryption can be mixed and as I don't decrypt I don't need the password.

Ah, fair enough, just checked an it is indeed possible to have both encrypted and non-encrypted entries in the same zip, I guess it might be possible to have different passwords for them too.

[darkms]

Awesome job!
Just left a small comment about encryption & password which I think would be important to validate in order to avoid security bugs and malformed archives. Happy to merge afterwards.