Allow DELETE as request method

[benjyhirsch]

It seems superagent's function for DELETE requests is called del rather than delete, which means that as it stands now, if the input Observable (i.e. the HTTP sink of main) emits an object like {url: http://foo.bar`, method: DELETE}, then the driver tries to call superagent[delete]`, which doesn't exist. I imagine that's not the desired behavior, though let me know if there's a reason that it is.

I'm new to this sort of thing, so let me know if there's something I should have done differently. It says the CI check failed for lack of credentials, but I imagine I'm not supposed to have whatever credentials those are. Is that what's supposed to happen?

[staltz]

Great, thanks.