GenerateDataKeyWithoutPlaintext now does not send KeySpec when sending NumberOfBytes

[stevegrossi]

Thanks for your work on ex_aws_kms, I hope this contribution is helpful and I'd be grateful for any feedback on it.

The Issue

I've run into an apparent issue when calling ExAws.KMS.generate_data_key_without_plaintext(number_of_bytes: 128): the response returned is {"ValidationException", "Please specify either number of bytes or key spec."}. That function adds a default "KeySpec" even when "NumberOfBytes" is provided in the options. Per the docs, AWS KMS expects a "KeySpec" or a "NumberOfBytes" but not both:

You must also specify the length of the data key. Use either the KeySpec or NumberOfBytes parameters (but not both).

The Fix

ExAws.KMS.generate_data_key avoids this issue by only adding the default "KeySpec" when "NumberOfBytes" is not given. This pull request applies that same approach to ExAws.KMS.generate_data_key_without_plaintext and adds a test to confirm it.

[artkay]

Thanks, @stevegrossi