Check fallbacks correctly in getHeaderVariant().

Check fallbacks correctly in getHeaderVariant().

Currently when users with Accept-Language: en accesses a zh site,
getVariantFallbacks() returns zh for it and zh is treated as their
preferred variant as header variant, making the configuration
setting $wgDefaultLanguageVariant ignored.

Per comments above getVariantFallbacks(), it returns the main
language code for an unknown variant (en in this case), so when
it's returning the main code, just ignore this fallback (when the
user is really sending Accept-Language: zh, it can be caught by
validateVariant() above and fallbacks are not consulted).

Change-Id: I187bdcd70b778883cc80ceeab005de3aca2e286a