check_scheme

# File lib/webrick/httpauth/authenticator.rb, line 40
      def check_scheme(req)
        unless credentials = req[@request_field]
          error("no credentials in the request.")
          return nil 
        end  
        unless match = /^#{@auth_scheme}\s+/.match(credentials)
          error("invalid scheme in %s.", credentials)
          info("%s: %s", @request_field, credentials) if $DEBUG
          return nil
        end
        return match.post_match
      end