Revert "Let pad mixin accept null to skip a side"

This reverts commit 22b451f.

Unfortely, now that bourbon is no longer a dependancy, this chage no longer
functions. If this functionality is required for your project, use bourbon and
import the `padding()` mixin.

app/assets/stylesheets/grid/_pad.scss

@@ -3,7 +3,7 @@
 /// Adds padding to the element.
 ///
 /// @param {List} $padding [flex-gutter()]
-///   A list of padding value(s) to use. Passing `default` in the list will result in using the gutter width as a padding value. Use a `null` value to “skip” a side.
+///   A list of padding value(s) to use. Passing `default` in the list will result in using the gutter width as a padding value.
 ///
 /// @example scss - Usage
 ///   .element {
@@ -16,10 +16,10 @@
 ///   }
 
 @mixin pad($padding: flex-gutter()) {
-  $vals: ();
+  $padding-list: null;
   @each $value in $padding {
     $value: if($value == 'default', flex-gutter(), $value);
-    $vals: append($vals, $value);
+    $padding-list: join($padding-list, $value);
   }
-  @include directional-property(padding, false, $vals);
+  padding: $padding-list;
 }

spec/neat/pad_spec.rb

@@ -25,29 +25,7 @@
 
   context "with argument (default)" do
     it "uses default gutter percentage" do
-      expect(".pad-shorthand-default").to have_rule("padding: 30px 2.35765% 10px")
-    end
-  end
-
-  context "with argument (null)" do
-    it "ignores rules with null values" do
-      ruleset = "padding-right: 10px; " +
-                "padding-left: 10px;"
-      bad_rule = "padding-top: null;"
-
-      expect(".pad-null-values").to have_ruleset(ruleset)
-      expect(".pad-null-values").to_not have_rule(bad_rule)
-    end
-  end
-
-  context "with argument (null default)" do
-    it "sets right and left paddings to gutter percentage" do
-      ruleset = "padding-right: 2.35765%; " +
-                "padding-left: 2.35765%;"
-      bad_rule = "padding-top: null;"
-
-      expect(".pad-null-and-default").to have_ruleset(ruleset)
-      expect(".pad-null-and-default").to_not have_rule(bad_rule)
+      expect(".pad-shorthand-default").to have_rule("padding: 30px 2.35765% 10px 2.35765%")
     end
   end
 end

test/pad.scss

@@ -15,11 +15,3 @@
 .pad-shorthand-default {
   @include pad(30px default 10px default);
 }
-
-.pad-null-values {
-  @include pad(null 10px null 10px);
-}
-
-.pad-null-and-default {
-  @include pad(null default);
-}