Delegate StubConnection#pubsub and StubConnection#config to ActionCable.server

Previously, `ActionCable::Channel::ConnectionStub` would
throw a `NoMethodError` when the channel under test calls
`stop_stream_for`, as shown in the example below:

```ruby
class ChatChannel < ActionCable::Channel::Base
  def subscribed
    stream_from "test"
  end

  def unsubscribed
    stop_stream_from "test"
  end
end

class ChatChannelTest < ActionCable::Channel::TestCase
  test "unsubscribe" do
    stub_connection

    subscribe

    # `unsubscribe` raises `NoMethodError` as `pubsub` does not
      exist on `ActionCable::Channel::ConnectionStub`
    unsubscribe

    assert_no_streams
  end
end
```

Calling `unsubscribe` causes an exception as `stop_stream_from` calls
`pubsub` when unsubscribing, which is not implemented in
`ActionCable::Channel::ConnectionStub`.

This commit fixes this issue by assigning the `ActionCable.server`
singleton to a `@server` instance variable in
`ActionCable::Channel::ConnectionStub`. This lets us delegate
the `config` and `pubsub` method calls to it.

actioncable/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Add a `@server` instance variable referencing the `ActionCable.server`
+    singleton to `ActionCable::Channel::ConnectionStub`
+
+    This lets us delegate the `pubsub` and `config` method calls
+    to the server. This fixes `NoMethodError` errors when testing
+    channel logic that call `pubsub` (e.g. `stop_stream_for`).
+
+    *Julian Foo*
+
 *   Added `health_check_path` and `health_check_application` config to
     mount a given health check rack app on a given path.
     Useful when mounting Action Cable standalone.

actioncable/lib/action_cable/channel/test_case.rb

@@ -47,9 +47,12 @@ def start_periodic_timers; end
     end
 
     class ConnectionStub
-      attr_reader :transmissions, :identifiers, :subscriptions, :logger, :config
+      attr_reader :server, :transmissions, :identifiers, :subscriptions, :logger
+
+      delegate :pubsub, :config, to: :server
 
       def initialize(identifiers = {})
+        @server = ActionCable.server
         @transmissions = []
 
         identifiers.each do |identifier, val|
@@ -59,7 +62,6 @@ def initialize(identifiers = {})
         @subscriptions = ActionCable::Connection::Subscriptions.new(self)
         @identifiers = identifiers.keys
         @logger = ActiveSupport::TaggedLogging.new ActiveSupport::Logger.new(StringIO.new)
-        @config = ActionCable::Server::Configuration.new
       end
 
       def transmit(cable_message)

actioncable/test/channel/test_case_test.rb

@@ -88,6 +88,10 @@ class StreamsTestChannel < ActionCable::Channel::Base
   def subscribed
     stream_from "test_#{params[:id] || 0}"
   end
+
+  def unsubscribed
+    stop_stream_from "test_#{params[:id] || 0}"
+  end
 end
 
 class StreamsTestChannelTest < ActionCable::Channel::TestCase
@@ -102,6 +106,13 @@ def test_stream_with_params
 
     assert_has_stream "test_42"
   end
+
+  def test_unsubscribe_from_stream
+    subscribe
+    unsubscribe
+
+    assert_no_streams
+  end
 end
 
 class StreamsForTestChannel < ActionCable::Channel::Base