Pass rack the absolute path of server's pid file

When running as a daemon, rack will cd to "/" and paths relative to the
root of the rails app aren't valid when rack is setting up.  Because of
this, "rails server -d" was failing silently when trying to write it's
pid file

Signed-off-by: José Valim <jose.valim@gmail.com>

railties/lib/rails/commands/server.rb

@@ -83,7 +83,7 @@ def default_options
         :environment => (ENV['RAILS_ENV'] || "development").dup,
         :daemonize   => false,
         :debugger    => false,
-        :pid         => "tmp/pids/server.pid"
+        :pid         => File.expand_path("tmp/pids/server.pid")
       })
     end
   end