including Mutex_m in a class doesn't initialise @_mutex

[feyeleanor]

Extending an object does work, including the module currently doesn't but the documentation says it does.

Hooking the included callback will fix this.

[znz]

include Mutex_m requires calling super in initialize if you write own initialize.

mutex_m/lib/mutex_m.rb

Lines 108 to 111 in 4a983ce

	def initialize(*args) # :nodoc:
	mu_initialize
	super
	end

So I think it is documentation problem.

[feyeleanor]

This worked back in Ruby 1.8 without needing mu_initialize to be called. I discovered this when playing with some of my old code from 2008 that worked at that time but doesn't work now without modification (I chose to use Extend instead as it's easier on my eyes).