Cannot set property 'maxAge' of undefined
After update to version "koa-session": "^3.1.0" ("koa": "~0.18.1")
Getting this error

```
TypeError: Cannot set property 'maxAge' of undefined
      at new Session (/Users/butuzgol/Playground/game/node_modules/koa-session/index.js:152:59)
      at Object.session (/Users/butuzgol/Playground/game/node_modules/koa-session/index.js:64:16)
      at Object.properties.(anonymous function).get (/Users/butuzgol/Playground/game/node_modules/koa-passport/lib/framework/request.js:99:21)
      at Function.initialize (/Users/butuzgol/Playground/game/node_modules/koa-passport/node_modules/passport/lib/middleware/initialize.js:50:12)
      at /Users/butuzgol/Playground/game/node_modules/co/index.js:135:8
      at Object.thunkToPromise (/Users/butuzgol/Playground/game/node_modules/co/index.js:134:10)
      at Object.toPromise (/Users/butuzgol/Playground/game/node_modules/co/index.js:118:55)
      at next (/Users/butuzgol/Playground/game/node_modules/co/index.js:98:29)
      at onFulfilled (/Users/butuzgol/Playground/game/node_modules/co/index.js:68:7)
      at /Users/butuzgol/Playground/game/node_modules/co/index.js:53:5
```

On  "koa-session": "^2.0.0" works fine
