Adding two missing possibilities for ValueListParameter

[frij-aws]

Adding two missing possibilities for ValueListParameter from from the SAP common vocabulary list

[phanak-sap]

Hi frij-aws.

Thx for pull request. Straightforward and simple.

1. I am quite surprised no test was actually need to be updated as well - clearly missing some automation here :)

2. Am I correct that we have in pyodata already hardcoded things that are SAP specific in a model.py, that tries to be covering Odata specification and adding SAP-only things ideally to different, isolated module like vendors/SAP.py? :)

Both are just open question, I am willing to merge as is.

[frij-aws]

To be honest I do not know what these constants are for or what they do, so I'm not 100% sure about the answers to your questions -- although I agree on (2) that it smells bad to have SAP-specific stuff in model.py. :)