Closes#943
* Datasette now requires httpx>=0.15
* Support OPTIONS without 500, closes#1001
* Added internals tests for datasette.client methods
* Datasette's own test mechanism now uses httpx to simulate requests
* Tests simulate HTTP 1.1 now
* Added base_url in a bunch more places
* Mark some tests as xfail - will remove that when new httpx release ships: #1005