Bug fixes and minor changes

• Fixed a bug where a IOException is incorrectly thrown for items backed by a non-seekable stream.
• ZipReader now throws new exception class Xceed.Zip.ReaderWriter.InvalidZipStructureException when invalid zip structure data is encountered. Although this is
• technically a breaking change, the exception class derives from ZipReaderException, which should preserve existing behavior.
• Fixed a bug where a IOException is incorrectly thrown for items backed by a non-seekable stream.
• Fixed a bug where zipped items encrypted with compatible encryption sometimes failed to decrypt if their last modified time was midnight and the FileTimes extra header was used.
• Increased speed of WinZipAES encryption and decryption when a large number of items are processed in an update or a read operation.