[scrapers] Fix processing xml files with incorrect suggested encoding or incorrect encoding in xml declaration

Reported on Russian XBMC forum.
If encoding is incorrect, xml can be parsed but data in xml can be broken. Common example is UTF-8 xml marked as Windows-1251 or vice versa. XML structure survive in such wrong conversion but scraped data is broken.
Without this patch, first tried encoding is used even if it break data in xml.
Second commit fix processing scraper generated XML as non-UTF-8. All XBMC internal text now is UTF-8, but some scrapers are incorrectly put another encoding in generated XML declaration which is now detected by XBMCTinyXML. In other places code for scraper processing already use TIXML_ENCODING_UTF8.