Call `MonitorMixin` initializer before use methods provided by `MonitorMixin`

`MonitorMixin` setup internal variables in `initialize`, so need to call
that before use.
`ActionController::Live::Buffer` only use methods that do not depend on those
internal variables, so works expertly.

But since Ruby 2.7, methods that were not originally the case also depend on
internal variables. As a result, some tests fail in Ruby 2.7.
https://buildkite.com/rails/rails/builds/65193#02e136eb-edea-4367-aee0-77e0a82f8531/990-1200

I'm not sure if this should be considered Ruby incompatible, but I fixed
this because its usage is wrong.

Related to [Feature #16255: Make `monitor.rb` built-in](https://bugs.ruby-lang.org/issues/16255)

actionpack/lib/action_controller/metal/live.rb

@@ -136,11 +136,11 @@ class Buffer < ActionDispatch::Response::Buffer #:nodoc:
       attr_accessor :ignore_disconnect
 
       def initialize(response)
+        super(response, SizedQueue.new(10))
         @error_callback = lambda { true }
         @cv = new_cond
         @aborted = false
         @ignore_disconnect = false
-        super(response, SizedQueue.new(10))
       end
 
       def write(string)