Connection: avoid sending backend oriented headers when doing requests to non-backend domains

[soxofaan]

This came up in discussions about signed URLs for batch job results (Open-EO/openeo-python-driver#50, Open-EO/openeo-api#380): batch job result assets could be hosted on different domain than the openEO backend (e.g. S3).
The Connection object should not include backend oriented headers (like Auth headers) when doing requests to these different domains. At the moment there is this blind assumption that all Connection requests are done to same backend.