Don't error when trying to write a message to a closed client take 2

This was first introduced in a2a46fd
but reverted because a new MessageBus release broke production and we
had to quickly revert before being able to isolate the commit. We have
since tested this commit in isolation in production and confirm that
MessageBus continues to work.

When a client is set to use chunked encoding by setting `use_chunked` to
true, trying to write a message to the client when the client has been
closed will raise an error. This commit changes it such that we don't
raise an error since the raised error doesn't do anything useful.

lib/message_bus/client.rb

@@ -278,7 +278,7 @@ def write_chunk(data)
       @async_response << data
       @async_response << postfix
       @async_response << NEWLINE
-    else
+    elsif @io
       @io.write(chunk_length.to_s(16) << NEWLINE << data << postfix << NEWLINE)
     end
   end

spec/lib/message_bus/client_spec.rb

@@ -99,6 +99,12 @@ def parse_chunk(data)
       chunk2.length.must_equal 0
     end
 
+    it "does not raise an error when trying to write a message to a closed client using chunked encoding" do
+      @client.use_chunked = true
+      assert(@client.closed?)
+      @client << MessageBus::Message.new(1, 1, "/test", "test")
+    end
+
     it "does not bleed data across sites" do
       @client.site_id = "test"