BaseStepDefinitionConverter form values not populated

Sorry about the previous pull request, I moved this change to its own branch.  Also thanks for the feedback on the parallel gateway change.  I will refactor locally to be more consistent with the code standards of changing the model.  I didn't see an obvious way to get the matching join gateway for a parallel gateway, even at the workflow definition conversion layer, but I will try harder!

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Fixed a minor bug in BaseStepDefinitionConverter in which the FormValues were being populated to a list, but never set back on the Property.
Therefore Form Properties, like lists/enums were not well formed, with
their corresponding values, in the resulting BPMN model.

Added unit test for the same.