Update stream.php

Changed isSupported() method, now it returns false if allow_url_fopen is not set.

Testcase:
=> Allow fopen() and deactivate "allow_url_fopen" and activate CURL in your php.ini. Browse to the Joomla!-Updater:
=> Cannot use a stream transport when "allow_url_fopen" is disabled.
=> Apply patch
=> Try again, Joomla! should use CURL now