Stringify and lowercase boolean params for api v1

Sending boolean vars as params to the requests lib results in URLs with capitalized values such as `private_networking=True`. That's not recognized as truth by DO's API v1.

It would be easy to walk around by passing strings as params but not as simple when using via Ansible since it casts params to boolean under the hood.

This is a simple patch that stringify and lowercase the booleans.

Thanks!