fix(subheader): Set styles to allow click/hover when stickied.

Previously, the inner DIVs of the subheader component were not set
to use `display: block`, so you could potentially have contents
that were inaccessible.

fixes #3932

src/components/subheader/subheader.scss

@@ -66,10 +66,12 @@ $subheader-sticky-shadow: 0px 2px 4px 0 rgba(0,0,0,0.16) !default;
  position: relative;
 
  .md-subheader-inner {
+ display: block;
  padding: $subheader-padding;
  }
 
  .md-subheader-content {
+ display: block;
  z-index: 1;
  position: relative;
  }