Fix Unit test

[GarrettBeatty]

Fix unit tests. not sure how previous dry runs never failed.

test Run Failed.
     Passed: 1
     Failed: 1
 Total time: 2.6469 Seconds
  AWSSDK.UnitTests.S3.NetFramework test failed with 1 error(s) (3.3s)
    C:\dev\repos\aws-sdk-net\sdk\src\Services\S3\Custom\Transfer\Internal\_async\MultipartUploadCommand.async.cs(156): error TESTERROR:
      Validation_HappyPath (1s 50ms): Error Message: Test method AWSSDK.UnitTests.MultipartUploadValidationTests.Validation_HappyPath threw except
      ion:
      Validation_HappyPath (1s 50ms): Error Message: Test method AWSSDK.UnitTests.MultipartUploadValidationTests.Validation_HappyPath threw      Validation_HappyPath (1s 50ms): Error Message: Test method AWSSDK.UnitTests.MultipartUploadValidationTests.Validation_HappyPath thr      Validation_HappyPath (1s 50ms): Error Message: Test method AWSSDK.UnitTests.MultipartUploadValidationTests.Validation_HappyPath threw except      ion:
      System.ArgumentNullException: Value cannot be null.
      Parameter name: source
      Stack Trace:

the reason it was failing is because we never mocked CompleteMultipartUploadResponse response object.

Testing

re-ran unit test and it passed

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license