Fix spawn_on_env vars leaking from server to app

Previously, variables specified by `spawn_on_env` would behave
inconsistently when the Spring server was created with one of those
variables set:

```
VAR_FROM_BOOT=before bin/rails ...
```

would cause future clients' apps to be booted with
`VAR_FROM_BOOT=before` set, even if the client is created without that
variable:

```
bin/rails ...
```

and additionally, Spring would hide that the app was _booted_ with that
var set by _unsetting_ it when the client attached to its app (making it
look like the correct ENV).

The issue was caused by how `spawn_on_env` was passed from client to
server to app:

- The client would slice `spawn_on_env` keys from its `ENV`, meaning
  `bin/rails ...` would have an empty `spawn_on_env` hash.
- The server (which has `VAR_FROM_BOOT` set), forks an app with
  `spawn_on_env` values set (none, because its an empty hash)
- The forked app inherits its parent's `ENV`, so it boots with
  `VAR_FROM_BOOT` set
- The client gets a forked app process, and cleans up environment
  variables from the server process

This is specifically an issue for `spawn_on_env` because the goal is to
have apps booted with different environments.

This commit fixes the issue by always passing all `Spring.spawn_on_env`
values from the client to the server (including `nil` ones) so that the
forked app will have those values cleared if necessary. `compact` is
added to the `SPRING_SPAWN_ENV` value because it is only used to display
the `spawn_on_env` values in `spring status` and showing all of the
`nil` values is noisy/unnecessary.

Author: skipkayhil

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"