Add back missing HTTP methods "connect", "trace"

[robingustafsson]

They seem to have gone missing when we switched from Otto to goja JS VMs (in https://github.com/loadimpact/k6/tree/v0.13.0).

See last implementation here (in JS at that time): https://github.com/loadimpact/k6/blob/v0.12.2/js/lib/k6/http.js

[cstyan]

OPTIONS in #451

we should find a way to properly test CONNECT and TRACE before adding them, see #464

[na--]

TRACE seems like a marginal HTTP method that doesn't deserve its own shortcut, people can just use the generic http.request('TRACE', ...) call for it.

CONNECT is a special method for HTTP proxies. Proxies are currently supported by k6, using the the semi-standardized HTTP_PROXY, HTTPS_PROXY and NO_PROXY environment variables (which we need to better document soon - grafana/k6-docs#74). And if that's not enough, more flexible support for proxies is tracked in another issue, #1045.

So, I think we can close this...