fixes #257 - added $format arg to date histogram (#259)

* fixes #257 - added $format arg to date histogram

* fixed typo

src/Aggregation/Bucketing/DateHistogramAggregation.php

@@ -28,19 +28,25 @@ class DateHistogramAggregation extends AbstractAggregation
      */
     protected $interval;
 
+    /**
+     * @var string
+     */
+    protected $format;
+
     /**
      * Inner aggregations container init.
      *
      * @param string $name
      * @param string $field
      * @param string $interval
      */
-    public function __construct($name, $field = null, $interval = null)
+    public function __construct($name, $field = null, $interval = null, $format = null)
     {
         parent::__construct($name);
 
         $this->setField($field);
         $this->setInterval($interval);
+        $this->setFormat($format);
     }
 
     /**
@@ -59,6 +65,14 @@ public function setInterval($interval)
         $this->interval = $interval;
     }
 
+    /**
+     * @param string $format
+     */
+    public function setFormat($format)
+    {
+        $this->format = $format;
+    }
+
     /**
      * {@inheritdoc}
      */
@@ -81,6 +95,10 @@ public function getArray()
             'interval' => $this->getInterval(),
         ];
 
+        if (!empty($this->format)) {
+            $out['format'] = $this->format;
+        }
+
         return $out;
     }
 }