Fix XML parse instruction detection.
* Now works with UTF-16 encoding (i.e. ignore null characters when reading binary stream)
* Encoding instruction was never being used even if detected. Recursion from 954f8bd001a4767f33206b0df70f8b26f8aadc69
* Remove intl.charset.default code, since intl.charset.default has been removed in Fx=>28 https://bugzilla.mozilla.org/show_bug.cgi?id=910192 (Haven't found a new way to get the fallback encoding from Firefox. intl.charset.fallback.override is not set by default