title: zeep SOAP client unexpected double base64 encoding
body: <p>Please help me understand why value in my request gets encoded to base64 twice:</p>
  
  <p>relevant parts of wsdl, that I use:</p>
  
  <pre><code>&lt;wsdl:types&gt;
        &lt;xsd:schema targetNamespace="http://www.blabla.com/"&gt;
           &lt;xsd:element name="createHeaderSecurityRequest" type="xsd:base64Binary"/&gt;
  
   &lt;wsdl:message name="createHeaderSecurityRequestMessage"&gt;
        &lt;wsdl:part element="tns:createHeaderSecurityRequest" name="parameters"/&gt;
     &lt;/wsdl:message&gt;
  
  &lt;wsdl:operation name="createHeaderSecurity"&gt;
           &lt;wsdl:input message="tns:createHeaderSecurityRequestMessage"/&gt;
           &lt;wsdl:output message="tns:createHeaderSecurityResponseMessage"/&gt;
           &lt;wsdl:fault message="tns:faultMessage" name="createHeaderSecurityFaultMessage"/&gt;
  /wsdl:operation&gt;
  </code></pre>
  
  <p>my code:</p>
  
  <pre><code>from zeep import Client
  message = 'my message, that needs to be encoded in base64 only once'
  wsdl = 'http://xxxxxxx?wsdl'
  client = Client(wsdl)
  request = client.create_message(client.service, 'createHeaderSecurity', message.encode())
  </code></pre>
  
  <p>what I get(message automatically encoded to base64 twice): request = </p>
  
  <pre><code>&lt;soap-env:Body xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"&gt;
     &lt;ns0:createHeaderSecurityRequest xmlns:ns0="http://www.blabla.com/"&gt;YlhrZ2JXVnpjMkZuWlN3Z2RHaGhkQ0J1WldWa2N5QjBieUJpWlNCbGJtTnZaR1ZrSUdsdUlHSmhjMlUyTkNCdmJteDVJRzl1WTJVPQ==&lt;/ns0:createHeaderSecurityRequest&gt;
  &lt;/soap-env:Body&gt;
  </code></pre>
  
  <p>what I want (message value encoded to base64 only once): request = </p>
  
  <pre><code>&lt;soap-env:Body xmlns:soap-env="http://schemas.xmlsoap.org/soap/envelope/"&gt;
     &lt;ns0:createHeaderSecurityRequest xmlns:ns0="http://www.blabla.com/"&gt;bXkgbWVzc2FnZSwgdGhhdCBuZWVkcyB0byBiZSBlbmNvZGVkIGluIGJhc2U2NCBvbmx5IG9uY2U=&lt;/ns0:createHeaderSecurityRequest&gt;
  &lt;/soap-env:Body&gt;
  </code></pre>
  
  <p>I will be very grateful for any help</p>
  
  <p>zeep 3.4.0
  python 3.7.3</p>