Fix adding static properties to model definition

[dimitor115]

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you written unit tests?
• Have you written unit tests that cover the negative cases (i.e.: if bad data is submitted, does the library respond properly)?
• This PR is associated with an existing issue?

Closing issues

Put closes #XXXX (where XXXX is the issue number) in your comment to auto-close the issue that your PR fixes.

Test plan
I've added unit tests that check if a static property is not included in the definition properties. (I also added a static property to TestClassModel.

[dimitor115]

closes #627

[WoH]

LGTM, thank you very much!