Fix exception from deleting a directory ZipEntry in a ZipFile

[benleedom]

ZipFile.FindExistingUpdate(ZipEntry) would always return -1 for directory ZipEntrys due to using GetTransformedFileName on all ZipEntrys. As a result, ZipFile.Delete(ZipEntry) would always throw an exception when passed a directory ZipEntry. The fix is to call GetTransformedDirectoryName for directory ZipEntrys.

I updated the test AddAndDeleteEntries in ZipFileHandling.cs to add and delete a directory; this test failed without the change to ZipFile.FindExistingUpdate.

I believe issue #290 is related.

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[piksel]

So, the consequence of this bug is that the entry name ends with a slash when created with ZipFile.AddDirectory, but not when it's searched for by ZipFile.Delete (which of course means it will not find it).

Test looks good. I'll try to release a bugfix update with this commit soon.