def normalize_keys(mapping)
      return mapping.inject({}) do |accu, pair|
        name, value = pair
        if Symbol === name
          name = name.to_s
        elsif name.respond_to?(:to_str)
          name = name.to_str
        else
          raise TypeError,
            "Can't convert #{name.class} into String."
        end
        accu[name] = value
        accu
      end
    end
def normalize_keys(mapping)
      return mapping.inject({}) do |accu, pair|
        name, value = pair
        if Symbol === name
          name = name.to_s
        elsif name.respond_to?(:to_str)
          name = name.to_str
        else
          raise TypeError,
            "Can't convert #{name.class} into String."
        end
        accu[name] = value
        accu
      end
    end
Generates a hash with string keys

 @param [Hash] mapping A mapping hash to normalize

 @return [Hash]
   A hash with stringified keys