title: Zeep setting my credentials values to NotSet
body: <p>I am trying to create a python soap client with zeep but run into the following problem:
  The credentials values that I set in the python dictionary are set by Zeep to NotSet and therefore I am getting: "Fault: Invalid credentials entered by user NotSet"
  I will paste the request that I am writing and then the request as sent by zeep where my clientId and password are changed to NotSet.</p>
  
  <pre><code>request_data={'language':'EN',
                  "identification": {"clientId":"something", "password":"something","customerKey":"" },...
  from zeep import Client
  from zeep.plugins import HistoryPlugin
  history = HistoryPlugin()
  wsdl='https://api.infinitehotel.com/gekko-front/ws/v2_4/AvailabilityService?wsdl'
  client = Client(wsdl,plugins=[history])
  response = client.service.hotelAvailability(**request_data)
  </code></pre>
  
  <p>The request that zeep is sending:</p>
  
  <pre><code>&lt;?xml version='1.0' encoding='utf-8'?&gt;
  &lt;soap-env:Envelope xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"&gt;&lt;soap-env:Body&gt;&lt;ns0:hotelAvailability xmlns:ns0="http://webservice.gekko-holding.com/v2_4"&gt;&lt;language&gt;EN&lt;/language&gt;&lt;identification clientId="NotSet" password="NotSet" customerKey=""/&gt;
  </code></pre>
  
  <p>As you can see it is modifying only the values of clientId and password to "NotSet"</p>
  
  <p>Can you please indicate a way to solve this problem?
  Thank you</p>