Failing spec for issue #562

Here's a failing spec for [#562](https://github.com/savonrb/savon/issues/562) which follows up on some regressions that seemed to have been reverted in PR [#39](https://github.com/savonrb/wasabi/pull/39). Hopefully this kicks off a bit of discussion on how best to move forward with the logic behind properly setting the message tag. 

In the case of issue #562, element may not work for setting the message_tag. [This section](https://github.com/savonrb/wasabi/blob/master/lib/wasabi/parser.rb#L273-280) of the parser in particular may need to be refactored a bit to accommodate this WSDL and others like it.

Here's the snippet from the WSDL:
```
<wsdl:message name="writeCaseEformData">
      <wsdl:part element="flt:FLEformFields" name="WriteCaseEform"/>
</wsdl:message>

<wsdl:operation name="writeCaseEformData">
       <wsdl:input message="fls:writeCaseEformData"/>
       <wsdl:output message="fls:writeCaseEformDataResponse"/>
       <wsdl:fault message="fls:serviceException" name="serviceException"/>
</wsdl:operation>
```