Backport of MAGETWO-59089 for Magento 2.1: Magento 2.1.1 Problem with change currency

Fixed issues:

- MAGETWO-59089 [Github]Magento 2.1.1 Problem with change currency magento#6746
(cherry picked from commit a9510d3)

Author: sdwright

lib/internal/Magento/Framework/Pricing/Render/PriceBox.php

@@ -86,7 +86,7 @@ public function getCacheKey()
      */
     protected function getCacheLifetime()
     {
-        return parent::hasCacheLifetime() ? parent::getCacheLifetime() : self::DEFAULT_LIFETIME;
+        return parent::hasCacheLifetime() ? parent::getCacheLifetime() : null;
     }
 
     /**

lib/internal/Magento/Framework/Pricing/Test/Unit/Render/PriceBoxTest.php

@@ -233,4 +233,17 @@ public function testGetRendererPool()
     {
         $this->assertEquals($this->rendererPool, $this->model->getRendererPool());
     }
+
+    /**
+     * This tests ensures that protected method getCacheLifetime() returns a null value when cacheLifeTime is not
+     * explicitly set in the parent block
+     */
+    public function testCacheLifetime()
+    {
+        $reflectionClass = new \ReflectionClass(get_class($this->model));
+        $methodReflection = $reflectionClass->getMethod('getCacheLifetime');
+        $methodReflection->setAccessible(true);
+        $cacheLifeTime = $methodReflection->invoke($this->model);
+        $this->assertNull($cacheLifeTime, 'Expected null cache lifetime');
+    }
 }