Fix #1473 unset > initial (#1597)

sass/components/dropdown.sass

@@ -29,7 +29,7 @@ $dropdown-divider-background-color: $border !default
     .dropdown-menu
       bottom: 100%
       padding-bottom: $dropdown-content-offset
-      padding-top: unset
+      padding-top: initial
       top: auto
 
 .dropdown-menu

sass/elements/form.sass

@@ -119,7 +119,7 @@ $help-size: $size-small !default
     max-height: 600px
     min-height: 120px
   &[rows]
-    height: unset
+    height: initial
   // Modifiers
   &.has-fixed-size
     resize: none
@@ -173,7 +173,7 @@ $help-size: $size-small !default
     &:not([multiple])
       padding-right: 2.5em
     &[multiple]
-      height: unset
+      height: initial
       padding: 0
       option
         padding: 0.5em 1em