add tests

Author: stas17

.idea/workspace.xml

@@ -17,10 +17,14 @@ public static function build($query)
         if (!is_array($query)) {
             return static::rawurlencode($query);
         }
-        return implode('&', array_map(function ($value, $key) {
+        $query = array_filter($query, function ($value) {
             if (null === $value) {
-                return null;
+                return false;
             }
+            return true;
+        });
+
+        return implode('&', array_map(function ($value, $key) {
             return static::encode($value, $key);
         }, $query, array_keys($query)));
     }

lib/Gitlab/HttpClient/Message/QueryStringBuilder.php

@@ -40,6 +40,17 @@ public function queryStringProvider()
             'iids%5B0%5D=88&iids%5B2%5D=86'
         ];
 
+        yield [
+            [
+                'source_branch'     => 'test_source',
+                'target_branch'     => 'test_master',
+                'title'             => 'test',
+                'assignee_id'       => null,
+                'target_project_id' => null,
+                'description'       => null
+            ],
+        ];
+
         //Boolean encoding
         yield [
             ['push_events' => false, 'merge_requests_events' => 1],