Fix encoding error when call REST API for the pages including local characters except English

[blurblah]

I changed encoding ISO-8859-1 to UTF-8 for XML generation. In case of including some local characters (in html_code), those characters are not changed properly because of ISO-8859-1 encoding when generate XML.

So we should use UTF-8 instead of ISO-8859-1. As we know, AChecker already uses UTF-8 encoding for HTML output.

[cindyli]

Please update the corresponding documentation for REST encoding: https://github.com/atutor/AChecker/blob/master/documentation/web_service_api.php#L325

Thanks for the contribution. We appreciate it.

[blurblah]

I removed ENT_IGNORE flag and tested it on some pages. And modified corresponding documentation, too.

While testing some pages (please check test.com using REST), I met undefined ndash entity error. So added ndash entity in RESTWebServiceOutput.class.php.

Please check my commit and add comment if you want.

[cindyli]

Thanks for the beautiful work, @blurblah.