kv_pair (TMail::Encoder)

    # File lib/action_mailer/vendor/tmail-1.1.0/tmail/encode.rb, line 288
        def kv_pair( k, v )
          return if v.nil?
          v = normalize_encoding(v)
          if token_safe?(v)
            add_text k + '=' + v
          elsif not CONTROL_CHAR === v
            add_text k + '=' + quote_token(v)
          else
            # apply RFC2231 encoding
            kv = k + '*=' + "iso-2022-jp'ja'" + encode_value(v)
            add_text kv
          end
        end