Fixes #1553

Unused $input-radius is now used as a default border-radius.

sass/elements/form.sass

@@ -47,6 +47,7 @@ $help-size: $size-small !default
   @extend %control
   background-color: $input-background-color
   border-color: $input-border-color
+  border-radius: $input-radius
   color: $input-color
   +placeholder
     color: $input-placeholder-color