Fixed \A and \z not being stripped from Sinatra

Sinatra 1.4.x (at least, might be earlier) uses \A and \z for its route regex. Turning `get "/ping"` into `/\A\/ping\z/` which the instrumentation does not strip properly because it's only looking for `^$`.

Which is what the changed line will fix so it instead returns "ping" not "\Aping\z"