Earlier today I removed the `s3://...` URL pseudo-schema from `detect_from_url`.  Getting rid of this "feature" was a good idea.  At that time, I started to remove the `:s3` symbol from `detect_from_symbol` but stopped myself.  It didn't seem like a good idea to force Formula writers to name an implementation class to specify a download strategy.  A minute ago, I went back again to take out `:s3`, but still couldn't bring myself to do it.  And I noticed a bunch of other strategies have symbolic names but no URL schemas, e.g., `:ssl3`, `:post`, and `:nounzip`.

I'll take out the `:s3` symbol if folks want me to, but I want to make the case one more time that keeping it in seems cleaner and more consistent with existing convention.