Request with empty array in nested element raises SOAPError "SOAP message structure is broken"

If the request contains a nested, empty array, the mentioned SOAPError is raised.

For example the following request works:

    <nested>
        <value>1</value>
        <value>1</value>
    </nested>

and produces `{nested: {value: [1,2]}}`, but the following request raises the error:

    <nested>
    </nested>

The pull requests avoids the error and produces `{nested: {}}`.

I also added SimpleCov configuration in spec_helper.