Single quotes are allowed for values during testing

Commercetools does not like single quotes around values. F.e. `createdAt >= '2019-09-09'` does not work, but does in our testing framework.

Low prio issue imo :P. PoC fix: https://github.com/labd/commercetools-python-sdk/compare/feature/bugfix-malformed-where?expand=1