Namespace prefix is required on first element when elementFormDefault=unqualified

[aleung]

This PR fixes an issue that the namespace prefix was missing on the first element in SOAP body.

In below example, this PR adds the tns in <tns:execFileParams>.

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:tns="urn:FileExecutor">
  <soap:Body>
    <tns:execFileParams>
      <fileName>PSTN</fileName>
      <arguments>abc</arguments>
      <base64EncodedStdIn>dGVzdA==</base64EncodedStdIn>
      <startDetached>true</startDetached>
    </tns:execFileParams>
  </soap:Body>
</soap:Envelope>

I have crosschecked the generated SOAP request in SoapUI-5.2.1 using the same WSDL.

[coveralls]

Coverage increased (+0.007%) to 93.144% when pulling 019a921 on aleung:missing_ns_prefix into 0544cc7 on vpulim:master.