Fix: aria-valuetext attribute added to the slider to read out scaleSt…

…epPrefix/scaleStepSuffix (fixes #211) (#212)

* Fix: aria-valuetext attribute added to the slider to read out prefix/suffix (fixes #211)

* conditional added to aria-valuetext

* update to add null to conditonal

templates/slider.jsx

@@ -178,6 +178,7 @@ export default function Slider (props) {
           <input className='slider__item-input js-slider-item-input'
             type='range'
             aria-label={ariaScaleName}
+            aria-valuetext={(scaleStepPrefix || scaleStepSuffix) ? `${scaleStepPrefix} ${selectedValue} ${scaleStepSuffix}` : null}
             value={selectedValue}
             min={_scaleStart}
             max={_scaleEnd}