handle symbols declared with %s

[theo-squadracer]

Close #1030

Symbols declared with %s[symbol] the same way :'symbol', the entire string is used as a symbol token.

[nobu]

Thank you for the report and the fix.
Please add a test like bellow to test/rdoc/test_rdoc_parser_ruby.rb too.

  def test_parse_percent_symbol
    content = '%s[foo bar]'
    util_parser content
    tk = @parser.get_tk
    assert_equal :on_symbol, tk[:kind]
    assert_equal content, tk[:text]
  end

[theo-squadracer]

Done, I don't know if I should do anything else