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

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.