mounted app.keys ignored

[idiomatic]

per @vinerz on koajs/koa#203:

... I want to set a isolated session path on /connect because this the only place I will use sessions, my app is a stateless API but I have to use sessions for Grant (Facebook & Twitter Sign-in). I don't want to share my API v1 keys with all other endpoints of the server.

var app = koa();

// first session handler
var connect = koa();
connect.keys = ['awesome-key'];
connect.use(session({
    store: redisStore()
}));

app.use(mount('/connect', connect));

// second session handler
var connect2 = koa();
connect2.keys = ['other-key'];
connect2.use(session({
    store: redisStore()
}));

app.use(mount('/connect2', connect2));

If both the parent app and mounted app have dissimilar keys arrays, the context may need a keys-merged keygrip instantiated upon mount descent.