lavf/http: fix cookie parsing.

lavf/http: fix cookie parsing.

The current code would use any unknown attribute-value pair
as the cookie value.
RFC 6265 states that the first key-value pair is the actual
cookie, and the attribute-value pairs only start after.

With the current code:
Set-Cookie: test=good_value; path=/; dummy=42
gives this:
Cookie: dummy=42
instead of this with the new code:
Cookie: test=good_value