Added getSource() function to populate source inline with the api.

Source isn't populated to the parent because it's transformed directly
to an array. Instead we're lazy loading the source when it's requested.

src/TagField.php

@@ -312,6 +312,21 @@ class TagField extends DropdownField
         return parent::setValue(array_filter($value));
     }
 
+    /**
+     * Gets the source array if required
+     *
+     * Note: this is expensive for a SS_List
+     *
+     * @return array
+     */
+    public function getSource()
+    {
+        if (is_null($this->source)) {
+            $this->setSource($this->getSourceList());
+        }
+        return $this->source;
+    }
+
     /**
      * {@inheritdoc}
      */