replace proxy settings in defaults with `null`.

The `mikeal/request` package already handles proxy support, including
environment variables `http_proxy`, `https_proxy`, `no_proxy`, and
variants. `npm` should let `request` take care of proxying based on
environment variables and only set proxy values if `.npmrc` has a proxy
specified. By passing a proxy to request, request will always use that
proxy setting and will not honor `no_proxy` settings. This will enable
the use of a private npm repository that is within a corporate firewall.

See https://github.com/npm/npm/issues/6524 for more details.
update config docs with new https_proxy and proxy default values
npm-registry-client@4.0.2

proxy via request and environment variables more transparently