Use #start_with? and #[] for speed

While the readability may be slightly worse, the speed improvement is
significant: Twice as fast when there's no leading "/" to remove, and
over 4 times as fast when there is a leading "/".

Benchmark:

    require 'benchmark/ips'

    def match(controller)
      if controller
        if m = controller.match(/\A\/(?<controller_without_leading_slash>.*)/)
          m[:controller_without_leading_slash]
        else
          controller
        end
      end
    end

    def start_with(controller)
      if controller
        if controller.start_with?('/'.freeze)
          controller[1..-1]
        else
          controller
        end
      end
    end

    Benchmark.ips do |x|
      x.report("match") { match("no_leading_slash") }
      x.report("start_with") { start_with("no_leading_slash") }

      x.compare!
    end

    Benchmark.ips do |x|
      x.report("match") { match("/a_leading_slash") }
      x.report("start_with") { start_with("/a_leading_slash") }

      x.compare!
    end

Result (Ruby 2.2.2):

    Calculating -------------------------------------
                   match    70.324k i/100ms
              start_with   111.264k i/100ms
    -------------------------------------------------
                   match      1.468M (± 7.1%) i/s -      7.314M
              start_with      3.787M (± 3.5%) i/s -     18.915M

    Comparison:
              start_with:  3787389.4 i/s
                   match:  1467636.4 i/s - 2.58x slower

    Calculating -------------------------------------
                   match    36.694k i/100ms
              start_with    86.071k i/100ms
    -------------------------------------------------
                   match    532.795k (± 4.7%) i/s -      2.679M
              start_with      2.518M (± 5.8%) i/s -     12.566M

    Comparison:
              start_with:  2518366.8 i/s
                   match:   532794.5 i/s - 4.73x slower

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -676,8 +676,8 @@ def use_relative_controller!
         # Remove leading slashes from controllers
         def normalize_controller!
           if controller
-            if m = controller.match(/\A\/(?<controller_without_leading_slash>.*)/)
-              @options[:controller] = m[:controller_without_leading_slash]
+            if controller.start_with?("/".freeze)
+              @options[:controller] = controller[1..-1]
             else
               @options[:controller] = controller
             end