Google\Spreadsheet\DefaultServiceRequest::addHeader PHP Method

    public function addHeader($name, $value)
    {
        $this->headers[$name] = $value;
        return $this;
    }

 public function testInitRequest()
 {
     $method = new \ReflectionMethod(DefaultServiceRequest::class, "initRequest");
     $method->setAccessible(true);
     $request = new DefaultServiceRequest("token");
     $request->addHeader("k", "v");
     $result = $method->invoke($request, "http://test");
     $this->assertTrue(is_resource($result));
     $result = $method->invoke($request, "spreadsheet");
     $this->assertTrue(is_resource($result));
 }