Fix CSV responses when index overriding with the call alias method

[megos]

CSV responses are implemented by overriding the index as below.

activeadmin/lib/active_admin/resource_controller/streaming.rb

Lines 12 to 17 in 2692838

	def index
	super do |format|
	format.csv { stream_csv }
	yield(format) if block_given?
	end
	end

If a user overrides the index using the alias method instead of the super, this implementation is ignored and CSV responses are not working.

controller do
  def index
    index! do |format| # Cause "No renderer defined for format: csv"
      ...
    end
  end
end

I add an alias method to make this implementation work anytime.