Fix the Facebook (Open graph) - joomla! Gzip problem

Facebook scraper gets only 40kb of data when scraping a page. If Gzip is enabled and the compressed page is more than that Facebook gets something that cannot decode (partial d/l). This P/R checks if user agent is facebookexternalhit/1.1 and if so disables the gzip
Merge branch 'facebook-gz' of https://github.com/dgt41/joomla-cms into facebook-gz