Fix saving version of model with JSON field

When a model uses `$casts = ['field' => 'array']` with a JSON field,
getAttributes() will return the JSON-encoded string, while
$model->fill() expects the input to contain the un-encoded JSON value.
So instead, use attributesToArray() to get the model data,
which will work properly with fill().

Fixes #43

src/Mpociot/Versionable/VersionableTrait.php

@@ -175,7 +175,7 @@ protected function versionablePostSave()
             $version->versionable_id   = $this->getKey();
             $version->versionable_type = get_class($this);
             $version->user_id          = $this->getAuthUserId();
-            $version->model_data       = serialize($this->getAttributes());
+            $version->model_data       = serialize($this->attributesToArray());
 
             if (!empty( $this->reason )) {
                 $version->reason = $this->reason;