The 'responsive' prop on the Button component has no effect

The Button component has the following prop:

/**
* If true, button scales to 100% width on small form factors.
*/
responsive: PropTypes.bool,

https://github.com/salesforce/design-system-react/blob/master/components/button/index.jsx#L177

This prop does not appear to have any effect when set to true (or false naturally).

This issue is demonstrated on the storybook example:
http://design-system-react-components.herokuapp.com/iframe.html?id=sldsbutton--doc-site-base-neutral&args=&viewMode=story

Looks like at some point the code conditionally adding the class was accidentally left out.

It should probably add the class slds-max-small-size--1-of-1 or some conditional variant of slds-button_stretch. In the past it was slds-max-small-button--stretch - this was removed in this commit https://github.com/salesforce/design-system-react/commit/a796c5b1fe1a967f1427a4fbd3ec4c9710d7592d on 22 Jul 2016:
https://github.com/salesforce/design-system-react/commit/a796c5b1fe1a967f1427a4fbd3ec4c9710d7592d#diff-1beac10e4cb18d36a1e2e3de7103d0f359da4b9b5748c851ce1c8061f1a803aeL143

Start in components/button/index.jsx at the responsive prop and compare the current class handling with the referenced commit. Reproduce the issue in the Storybook Button example, then verify that setting responsive changes the small-form-factor width behavior and that leaving it false has no effect.