Use json() property of responses to the proxy requests

[pablo-de-andres]

Currently, the requests sent to the proxy are loaded via json. Example from newTransformation capability:

 json.loads(
                self._client.post(
                    self._proxy_path("newTransformation"), json=new_transformation
                )
)

This gives an error, since the call to self._client.post() is returning a Response object. This can easily be fixed by replacing the call to json.loads() to the json() method of the response:

self._client.post(
                    self._proxy_path("newTransformation"), json=new_transformation
).json()