Spinner color inside a button

[vdineva]

When I use a spinner inside a button (https://magnetic.cisco.com/0a43ab5cd/p/56608b-button/t/330d12)

the color of the spinner is grey:

and in the Magnetic specs it's white:

() => {
  return (
      <AButton disabled medium>
          <ASpinner size="small"  />
          Submit
     </AButton> 
    );
}

Also, it would be nice if magna-react handles the margin to it's right

[androche]

Completed in #359