Mix of express/koa with passport in both breaks
If youve an koa app, with passport and mount an express app in there, also with passport, this lib produces errors.
This is happens cause of manipulation the global passport module, instead of just modify an instance.

So to point it out: https://github.com/rkusa/koa-passport/blob/8bf9697e53dcdf374a067148c103dfce3736eee3/lib/index.js#L12
This line is causing issues.

every passport used after the import of koa-passport will use the framework. This is maybe wanted in 99% of the use cases in my case this is breaking everything.

In my app the Koa auth happens first and is also successful, afterwards express is taking over and fails cause it also uses the koa-passport lib. Since i tried to only instantiate on instance with koa-passport (ive only possibility to edit the koa app, not the express) it should use the framework also only in this instance.

`const Instance = new passport.KoaPassport()`

Error:
```
TypeError: Cannot read property 'hasOwnProperty' of undefined
    at passportInitialize (******/node_modules/koa-passport/lib/framework/koa.js:27:18)
```

if i comment out this line, all works as i wanted it to work
