fix(list): not working correctly when list item is used as a button

Based on our selector for `mat-list-item`, we allow it to be a `button`, however we don't do the necessary resets for it to be usable.

Author: crisbeto

src/lib/list/list.scss

@@ -43,9 +43,11 @@ $mat-list-item-inset-divider-offset: 72px;
   height: $base-height;
   -webkit-tap-highlight-color: transparent;
 
-  // Override the user agent styling if the list item is a button.
-  width: 100%;
+  // Button resets.
+  border: none;
+  background: none;
   padding: 0;
+  width: 100%;
 
   .mat-list-item-content {
     display: flex;