alpha should not be required on hsb()

_hsv.scss

@@ -49,7 +49,7 @@
     @return hsv($h, $s, $v, $a);
 }
 
-@function hsb($h, $s, $b, $a) {
+@function hsb($h, $s, $b, $a: 1) {
     @return hsv($h, $s, $b, $a);
 }