Close all outstanding responses on client close

[lovelydinosaur]

Client close currently closes off any keep-alive connections.
Actually the behavior would be more neatly bounded if we forcibly close off all connections.

with SyncClient() as client:
    response = client.get(..., stream=True)
response.read()  # This should raise an error.

[jhtimmins]

@florimondmanca is this still up for grabs? And if so, would it make a good first issue?

[florimondmanca]

Hi @jhtimmins! There's already a PR for this over at #322, but unfortunately it seems to have slipped through.

So feel free to take it up. I don't think the fix will be trivial, but you can take a crack at it and see where you end up. :)

[jhtimmins]

Thanks @florimondmanca! Sounds great. I'll do that.

[lovelydinosaur]

Obsoleted by the more tightly bounded stream() API. (Which ensures that connections can only be held open within a stream() context block)