Fix broken ServerTest with Capybara 3.0.0

It seems that it is no longer possible to specify the value held by
`Capybara.server` as sever.

Ref: ba7674086c

Gemfile.lock

@@ -162,13 +162,13 @@ GEM
     bunny (2.6.6)
       amq-protocol (>= 2.1.0)
     byebug (9.0.6)
-    capybara (2.15.1)
+    capybara (3.0.0)
       addressable
       mini_mime (>= 0.1.3)
-      nokogiri (>= 1.3.3)
-      rack (>= 1.0.0)
-      rack-test (>= 0.5.4)
-      xpath (~> 2.0)
+      nokogiri (~> 1.8)
+      rack (>= 1.6.0)
+      rack-test (>= 0.6.3)
+      xpath (~> 3.0)
     childprocess (0.7.1)
       ffi (~> 1.0, >= 1.0.11)
     chromedriver-helper (1.1.0)
@@ -298,7 +298,7 @@ GEM
     mime-types-data (3.2016.0521)
     mimemagic (0.3.2)
     mini_magick (4.8.0)
-    mini_mime (0.1.4)
+    mini_mime (1.0.0)
     mini_portile2 (2.3.0)
     minitest (5.11.3)
     minitest-bisect (1.4.0)
@@ -339,7 +339,7 @@ GEM
     pg (1.0.0-x86-mingw32)
     powerpack (0.1.1)
     psych (3.0.2)
-    public_suffix (3.0.1)
+    public_suffix (3.0.2)
     puma (3.9.1)
     puma (3.9.1-java)
     que (0.14.0)
@@ -482,8 +482,8 @@ GEM
     websocket-driver (0.6.5-java)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.2)
-    xpath (2.1.0)
-      nokogiri (~> 1.3)
+    xpath (3.0.0)
+      nokogiri (~> 1.8)
 
 PLATFORMS
   java

actionpack/test/dispatch/system_testing/server_test.rb

@@ -5,10 +5,6 @@
 require "action_dispatch/system_testing/server"
 
 class ServerTest < ActiveSupport::TestCase
-  setup do
-    @old_capybara_server = Capybara.server
-  end
-
   test "port is always included" do
     ActionDispatch::SystemTesting::Server.new.run
     assert Capybara.always_include_port, "expected Capybara.always_include_port to be true"
@@ -27,6 +23,6 @@ class ServerTest < ActiveSupport::TestCase
   end
 
   teardown do
-    Capybara.server = @old_capybara_server
+    Capybara.server = :default
   end
 end