&proxy with no value doesn't work

[diffbot-dan-urman]

The docs page for "Error Downloading URL Error" (old, new) indicates that proxies can be added via the &proxy argument by itself with no value.

As discovered in a customer support issue this does not seem to work (though perhaps it used to). I haven't gotten anything definitive from any developer on the intent here, so let's just go ahead and update the docs to the form of this that actually works, which is &useProxy=default.

(&proxy=default works as well but useProxy is probably better since we have a couple of proxy-related arguments to discuss in the API docs per #12 and it's better to avoid ambiguity.)