Easy access helper for JSON.parse in ruby?

To convert any object into a JSON, we have foo.to_json helper in ruby which returns a string. Similarly, to convert it back to its original datatype, we have to use JSON.parse(json_string), which does not look as good as foo.to_json (IMHO).

So I tried to implement a helper function for that in ruby using the following code:

class String
  def parse_json
    JSON.parse(self)
  end
end

But none of the languages I know of has such a helper method in the String class. This makes me wonder, is it a bad pattern to write code like this? The String instance should not be responsible for decoding itself into some other type of object that is present in the JSON?