If initialize is overriden and super is not called inject fails

[krisleech]

Because @injections is never set.

We can capture this and report a nice message here if @injections is nil.

Or, we can not initialize @injections in the constructor favouring lazy initialization in the injections method:

def injections
  @injections ||= Injections.new
end

The problem with the above is that, currently, injections returns a immutable value and the method is not used internally, @injections is.