fix(button): hover styles being applied to disabled buttons (#909)

Fixes the button hover styles being applied, even if a button is disabled.

Fixes #866.

src/components/button/button.scss

@@ -4,11 +4,17 @@
 @import 'button-base';
 
 // TODO(kara): Replace attribute selectors with class selectors when possible
-[md-button] {
+[md-button], [md-icon-button] {
   @extend %md-button-base;
 
+  // Only flat buttons and icon buttons (not raised or fabs) have a hover style.
+  &:hover {
+    // Use the same visual treatment for hover as for focus.
+    @include md-button-focus();
+  }
+
   &[disabled]:hover {
-    &.md-primary, &.md-accent, &.md-warn, &:hover {
+    &.md-primary, &.md-accent, &.md-warn, &::after {
       background-color: transparent;
     }
   }
@@ -60,12 +66,6 @@
   z-index: 1;
 }
 
-// Only flat buttons and icon buttons (not raised or fabs) have a hover style.
-[md-button]:hover, [md-icon-button]:hover {
-  // Use the same visual treatment for hover as for focus.
-  @include md-button-focus();
-}
-
 // Applies a clearer border for high-contrast mode (a11y)
 @media screen and (-ms-high-contrast: active) {
   .md-raised-button, .md-fab, .md-mini-fab {