Add to the mapping properties instead of overwriting them

The properties key of the mappings key in the mapping array already has
values provided by the ElasticPress mapping. In the old situation, that
entire key was overwritten, instead of adding the custom mappings to the
properties.

src/Base/ElasticPress/ElasticPress.php

@@ -232,20 +232,19 @@ protected function formatOutputForElasticsearch(array $item): array
      */
     public function addMappings(array $mapping): array
     {
-        $mapping['mappings']['properties'] = [
-            'expired' => [
-                'type'       => 'object',
-                'properties' => [
-                    'on' => [
-                        'type'    => 'object',
-                        'enabled' => 'false'
-                    ]
+        $mapping['mappings']['properties']['expired'] = [
+            'type'       => 'object',
+            'properties' => [
+                'on' => [
+                    'type'    => 'object',
+                    'enabled' => 'false'
                 ]
             ],
-            'post_date_gmt'         => [
-                'type'   => 'date',
-                'format' => 'yyyy-MM-dd HH:mm:ss',
-            ],
+        ];
+
+        $mapping['mappings']['properties']['post_date_gmt'] = [
+            'type'   => 'date',
+            'format' => 'yyyy-MM-dd HH:mm:ss',
         ];
 
         return $mapping;