applehttp: Fix the key check in handle_variant_args

The key string is supposed to contain the equals character,
too.  Since the checked string was wrong, and the return value
check was wrong too, it incorrectly seemed to work right before.

Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 2b0decf60ba6dd5eec16e88d4b816e56108a2935)
