UploadHandler has incorrect propTypes for abort and upload

[matrangam]

When using the UploadHandler with renderProps, as described in the example, all usage of the component results in the following errors in the browser:

Warning: Failed prop type: The prop abort is marked as required in UploadHandler, but its value is undefined.

Warning: Failed prop type: The prop upload is marked as required in UploadHandler, but its value is undefined

          <UploadHandler file={selectedFile} autoStart={true}>
            {({ upload, abort }) => {...}
          </UploadHandler>

Looking to see if we can adjust the PropTypes to be less strict to allow for this type of usage.