Fix double underscore query parameters properly

[JakubFrejlach]

In #17 the issue with double underscore parameters was fixed in a kinda hacky way (replacing all the double underscores with the single ones on the bindings side) however it would probably require a nicer way of fixing it in the form of changing the jinja templates of the bindings. Changes required are however very extensive.

These changes should be probably done together with the updating the bindings to the newer version of OpenAPI Python Client which is tracked by the #18