Fix spawn_on_env vars leaking from server to app

lib/spring/application_manager.rb

@@ -102,7 +102,7 @@ def start_child(preload = false)
             "RACK_ENV"            => app_env,
             "SPRING_ORIGINAL_ENV" => JSON.dump(Spring::ORIGINAL_ENV),
             "SPRING_PRELOAD"      => preload ? "1" : "0",
-            "SPRING_SPAWN_ENV"    => JSON.dump(spawn_env),
+            "SPRING_SPAWN_ENV"    => JSON.dump(spawn_env.compact),
             **spawn_env,
           },
           "ruby",

lib/spring/client/run.rb

@@ -254,7 +254,9 @@ def default_rails_env
       end
 
       def spawn_env
-        ENV.slice(*Spring.spawn_on_env)
+        Spring.spawn_on_env.to_h do |key|
+          [key, ENV[key]]
+        end
       end
     end
   end

test/support/acceptance_test.rb

@@ -663,6 +663,18 @@ def exec_name
         assert_success %(bin/rails runner 'p ENV.key?("FOO")'), stdout: "false"
       end
 
+      test "spawn_on_env variables are cleared when unset" do
+        File.write(app.spring_client_config, "Spring.spawn_on_env << 'VAR_FROM_BOOT'")
+        File.write(app.application_config, "#{app.application_config.read}\nRails.configuration.x.var_from_boot = ENV['VAR_FROM_BOOT']")
+
+        app.env["VAR_FROM_BOOT"] = "before"
+        assert_success %(bin/rails runner 'p Rails.configuration.x.var_from_boot.inspect'), stdout: "before"
+
+        app.env.delete "VAR_FROM_BOOT"
+
+        assert_success %(bin/rails runner 'p Rails.configuration.x.var_from_boot.inspect'), stdout: "nil"
+      end
+
       test "Kernel.raise remains private" do
         expr = "p Kernel.private_instance_methods.include?(:raise)"
         assert_success %(bin/rails runner '#{expr}'), stdout: "true"