Sending a custom header with ZEND_HTTP_CLIENT

How can I send a custom header with the ZEND_HTTP_CLIENT. I am trying to send a variable key with a certain value, that I will check later for authenticity

I've tried this

       $client = new Zend_Http_Client('http://localhost/v3/files/');
       $client->setHeaders('Content-type','multipart/form-data');
       $client->setHeaders('key','XXXxXXXXXXXXXXXXXX');

       $client->setParameterPost('document_id', $id);
       $client->setParameterPost('type_id', $docType['type']);
       $client->setParameterPost('file', $form->file);
       $response = $client->request(Zend_Http_Client::POST);

and this

  $client = new Zend_Http_Client('http://localhost/v3/files/');
   $client->setHeaders(array(
      'Content-type','multipart/form-data',
      'key','XXXxXXXXXXXXXXXXXX'));

       $client->setParameterPost('document_id', $id);
       $client->setParameterPost('type_id', $docType['type']);
       $client->setParameterPost('file', $form->file);
       $response = $client->request(Zend_Http_Client::POST);

but it doesnt seem to work. It says key is not a valid type. I want to send a custom header like this (similar to what happens when you set headers with the Postman client). Is this possible?

Try with add a config param like this:

 $client = new Zend_Http_Client('http://localhost/v3/files/', array('strict' => false));