class ActiveSupport::HashWithIndifferentAccess

counters[:zoo] # => nil
counters[:foo] # => 1
counters['foo'] # => 1

counters[:foo] = 1
counters = ActiveSupport::HashWithIndifferentAccess.new

either a string or a symbol:
Same as Hash#[] where the key passed as argument can be

def [](key)
  super(convert_key(key))
end