(url-scheme url or http-url) -> string or #f
Returns the scheme field of the given <url> or <http-url>.
Example:
>> testurl :: [http-url #f "www.google.com" 80 ("search") [dict ("client" . "firefox-a") ("ie" . "utf-8") ("oe" . "utf-8") ("q" . "MOSREF Ephemeral Security") ("rls" . "org.mozilla:en-US:official") ("start" . "0")] #f] >> (url-scheme testurl) :: "http"