url: support suppressing Accept header Sending this header automatically could interfere with future testing and client behavior. Let's add a knob to disable the behavior. We don't have a control for User-Agent because urllib will send it if we don't set something. I don't feel like hacking into the bowels of urllib to figure out how to suppress that. UA shouldn't be used for anything meaningful. So it shouldn't pose any problems beyond non-determinism (since the header has the Mercurial version in it). Differential Revision: https://phab.mercurial-scm.org/D2843