Cast toolbar height prop to an int

If you passed height as a string, it would have the system bar's
height appended to it when setting the content height, typically
multiplying the correct padding by 10

See https://codepen.io/anon/pen/XeEdmq?editors=1010

src/components/VToolbar/VToolbar.js

@@ -51,7 +51,7 @@ export default {
 
   computed: {
     computedHeight () {
-      if (this.height) return this.height
+      if (this.height) return parseInt(this.height)
       if (this.dense) return this.heights.dense
       if (this.prominent ||
         this.$vuetify.breakpoint.mdAndUp