Overrides the indifferent access of a Mash to keep keys in the original format given to the Mash.

@example

  class KeepingMash < Hashie::Mash
    include Hashie::Extensions::Mash::KeepOriginalKeys
  end

  mash = KeepingMash.new(:symbol_key => :symbol, 'string_key' => 'string')
  mash.to_hash  #=> { :symbol_key => :symbol, 'string_key' => 'string' }
  mash['string_key'] == mash[:string_key]  #=> true
  mash[:symbol_key] == mash['symbol_key']  #=> true