Fix markdown autolink issues.

* Rescue C auto-linked content raised ArgumentError.

require 'redcarpet'
markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true, :space_after_headers => true)
markdown.render 'https://github.com码';

=> "<p><a href=\"https://github.com%E7\">https://github.com\xE7</a>\xA0\x81</p>\n"

* Incorrectly autolink when UTF-8 character stick with the end of URL

require 'redcarpet'
markdown = Redcarpet::Markdown.new(Redcarpet::Render::HTML, :autolink => true, :space_after_headers => true)
markdown.render '（https://github.com）';

=> "<p>（<a href=\"https://github.com%EF%BC%89\">https://github.com）</a></p>\n"