title: Pass oauth header in soap call with erlang detergent library
body: <p>I am using <a href="https://github.com/devinus/detergent" rel="nofollow">detergent</a> to make soap call to salesforce soap api.
  I want to call its function call/4 but it fails:</p>
  
  <pre><code>:detergent.call("metadata.wsdl", "describeMetadata", ["37.0"], [{'sessionId',token.access_token}])
  
  
  ** (exit) an exception was raised:
  ** (FunctionClauseError) no function clause matching in :erlsom_write.processAnyAttributes/4
      src/erlsom_write.erl:501: :erlsom_write.processAnyAttributes('ok', [], [{:ns, 'http://schemas.xmlsoap.org/soap/envelope/', 'soap', :unqualified}, {:ns, 'http://soap.sforce.com/2006/04/metadata', 'p', :qualified}, {:ns, 'http://www.w3.org/2001/XMLSchema', 'xsd', :qualified}], {[{'soap', 'http://schemas.xmlsoap.org/soap/envelope/'}], 0})
      src/erlsom_write.erl:325: :erlsom_write.processAlternativeValue/8
  </code></pre>
  
  <p>What is the expected format of the header syntax to pass token?</p>