some proxies require `request_fulluri` set to true [see #36]

at least polipo won't work for plain HTTP URLs (HTTPS strangely also works without `request_fulluri`..?)

see https://git.tt-rss.org/git/tt-rss/pulls/36

include/functions.php

@@ -496,6 +496,7 @@
 			}
 
 			if (defined('PROXY')) {
+				$context_options['http']['request_fulluri'] = true;
 				$context_options['http']['proxy'] = PROXY;
 			}