Invalid config maxAge type not handled
A simple mistake from developer that is uncaught from koa session makes it really hard to debug. Suggest to add assert to config types.

For example, wrong maxAge type results in cookie expires/max-age not set correctly
const config = {
  maxAge: '86400000',
};

<img width="615" alt="screen shot 2018-06-16 at 11 19 06 pm" src="https://user-images.githubusercontent.com/5749127/41499921-ae43757c-71bb-11e8-903c-804e908ab6d4.png">


did a simple fix at #125 