Default returns the user-agent for Dingo's outbound HTTP requests. A
non-empty DINGO_HTTP_USER_AGENT that is a valid header value overrides it;
otherwise it names Dingo's version and the platform on which it was built.
Set sets Default as the request's user-agent unless the request already
carries a User-Agent header. A present but empty header is kept, because
net/http then sends no User-Agent at all.