Set my desiredCompressionMethod
This is the method that will be used to write.
Answers prior desiredCompressionMethod.
Only CompressionDeflated or CompressionStored are valid arguments.
Changing to CompressionStored will change my desiredCompressionLevel
to CompressionLevelNone; changing to CompressionDeflated will change my
desiredCompressionLevel to CompressionLevelDefault.