code: def stringify_headers headers
        headers.inject({}) do |result, (key, value)|
          if key.is_a? Symbol
            key = key.to_s.split(/_/).map(&:capitalize).join('-')
          end
          if 'CONTENT-TYPE' == key.upcase
            result[key] = maybe_convert_extension(value.to_s)
          elsif 'ACCEPT' == key.upcase
            # Accept can be composed of several comma-separated values
            if value.is_a? Array
              target_values = value
            else
              target_values = value.to_s.split ','
            end
            result[key] = target_values.map { |ext|
              maybe_convert_extension(ext.to_s.strip)
            }.join(', ')
          else
            result[key] = value.to_s
          end
          result
        end
      end
docs: Return a hash of headers whose keys are capitalized strings
  
   BUG: stringify_headers does not fix the capitalization of headers that
   are already Strings. Leaving this behavior as is for now for
   backwards compatibility.
   https://github.com/rest-client/rest-client/issues/599