fix(core): no longer strip slashes on $_FILES and $_SERVER

When magic_quotes_gpc is enabled on your server file uploads would fail.

The Elgg_HTTP_Request class should only undo the magic quotes on the GPC
(GET, POST and COOKIES) super global not on other super globals (SERVER
and FILES)

Fixes #6777
Merge pull request #6791 from jeabakker/elgg-http-request-magic-quotes-gpc

fix(core): no longer strip slashes on $_FILES and $_SERVER