Replaced explicit \VuFind\Cart calls with cart() view helper.

themes/blueprint/templates/header.phtml

 <? $account = $this->layout()->account; ?>
 <a id="logo" href="<?=$this->url('home')?>"></a>
 <div id="headerRight">
-  <? $cart = \VuFind\Cart::getInstance(); if ($cart->isActive()): ?>
+  <? $cart = $this->cart(); if ($cart->isActive()): ?>
     <div id="cartSummary" class="cartSummary">
       <a id="cartItems" title="<?=$this->transEsc('View Book Bag')?>" class="bookbag" href="<?=$this->url('cart-home')?>"><strong><span><?=count($cart->getItems())?></span></strong> <?=$this->transEsc('items')?> <?=$cart->isFull() ? '(' .  $this->transEsc('bookbag_full') . ')' : ''?></a>
       <a id="viewCart" title="<?=$this->transEsc('View Book Bag')?>" class="viewCart bookbag offscreen" href="<?=$this->url('cart-home')?>"><strong><span id="cartSize"><?=count($cart->getItems())?></span></strong> <?=$this->transEsc('items')?><span id="cartStatus"><?=$cart->isFull() ? $this->transEsc('bookbag_full') : '&nbsp;'?></span></a>

themes/blueprint/templates/layout/layout.phtml

@@ -22,7 +22,7 @@
         $this->headScript()->prependScript("path = '" . rtrim($this->url('home'), '/') . "';");
 
         // Deal with cart stuff:
-        $cart = \VuFind\Cart::getInstance();
+        $cart = $this->cart();
         if ($cart->isActive()) {
             $this->headScript()->appendFile("jquery.cookie.js");
             $this->headScript()->appendFile("cart.js");