Check for presence of val in data array

In model. I think this simpler patch should accomplish the same as
issue 44.

Author: Ian Barber

src/Google/Model.php

@@ -46,7 +46,12 @@ public function __get($key)
     $keyTypeName = $this->keyType($key);
     $keyDataType = $this->dataType($key);
     if (isset($this->$keyTypeName) && !isset($this->processed[$key])) {
-      $val = $this->data[$key];
+      if (isset($this->data[$key])) {
+        $val = $this->data[$key];
+      } else {
+        $val = null;
+      }
+      
       if ($this->isAssociativeArray($val)) {
         if (isset($this->$keyDataType) && 'map' == $this->$keyDataType) {
           foreach ($val as $arrayKey => $arrayItem) {