Upstream: fixed inheritance of proxy_store and friends.

The proxy_store, fastcgi_store, scgi_store and uwsgi_store were inherited
incorrectly if a directive with variables was defined, and then redefined
to the "on" value, i.e. in configurations like:

    proxy_store /data/www$upstream_http_x_store;

    location / {
        proxy_store on;
    }