feature (xml key): allow in first level for rpc

[jglongaron]

• allow xmlKey in first level for rpc

Problem

Using xmlKey ('$xml') in the first level of args

var soap = require('soap');
  var url = 'http://example.com/wsdl?wsdl';
  var args = {$xml: '<action>1</action>'};

  soap.createClient(url, {}, function(err, client) {
      client.MyFunction(args, function(err, result) {
          ...
      });
  });

Request includes "$xml" tag instead of unescaped text

...<soap:body><MyFunction><$xml>&lt;tns:action&gt;&lt;...&lt;tns:/action&gt;&lt;</$xml></MyFunction></soap:body>...

New Request

...<soap:body><MyFunction><action>1</action></MyFunction></soap:body>...