XMLHttpRequest test fixes 2

- getresponseheader-special-characters.htm: horizontal ellipsis (…) can be specified by UTF-8 hex value string in PHP but in JavaScript UTF-8 charset might have to be specified \uXXXX format rather than \xXX which denotes Latin-1 charset. UAs actually interpret "\xE2\x80\xA6" as "â¦". getResponseHeader() in Firefox returns "â¦" instead of "\u2026". It may be decoding the unicode charset in the header field value with Latin-1 charset rule?
- open-after-setrequestheader.htm: corrected the query string of the request URL to make it work correctly with the inspect-headers.php which requires "filter_name" instead of "filter".