This PR fixes#4615
From the issue :
> One thing we might consider is merging and showing a warning for keys not defined in exportPathMap
The behaviour after this PR is the following :
```js
// next.config.js
module.exports = {
exportPathMap: () => ({
'/': { page: '/', query: { a: 'blue' } }
})
}
```
| url called | `ctx.query` | warning ? |
|-|-|-|
| `/` | `{ a: 'blue' }` | |
| `/?a=red` | `{ a: 'blue' }` | |
| `/?b=green` | `{ a: 'blue', b: 'green' }` | `... parameter 'b' missing in exportPathMap` |
Is that the expected behaviour ? If not, I'll update the PR to shape the expected behavior.
