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) {