Need to add Attributes for child items.

[ryanaghdam]

The service we are using requires attributes to be specified in child items. For example, the following is expected:

<env:Envelope>
  <env:Body>
    <ins16:updateRecipientProfile>
      <ins16:masterProperty name="email">
          <ins16:value>me@a-place.com</ins16:value>
      </ins16:masterProperty>
    </ins16:updateRecipientProfile>
  </env:Body>
</env:Envelope>

i.e. The attribute doesn't go on the method call itself, but one of the children in the messages hash passed to call.

Related to #361.

[ryanaghdam]

This can be achieved by prefixing keys with @, as per Gyoku documentation.