Object returned by Invoke-vROWorkflow can not be pipelined to other cmdlets

[voroniys]

Steps to reproduce

Invoke-vROWorkflow -Id $vro_wf_id | Get-vROWorkflowExecutionState

Expected behavior

Should get an execution state

Actual behavior

Get-vROWorkflowExecutionState failed with error.

Cause and solution

The issue cause by the wrong data in Execution property. It is
a - array
b - full URL

The solution is in pull request #14

[jonathanmedd]

@voroniys Thanks a lot for this! Have merged your PR.