bug-fix: Error computing build: undefined

I tried running `node server.js --computeForPath='index.html'` from `react-page` and got `ERROR computing build: undefined`.

Looking at the code, `exports.provide` has the following check:

```javascript
if (req.method !== 'GET') {
  return next();
}
```
and `compute()`'s `req` is `{url: mockRequestedURL}`. It also later uses `res.setHeader`. 

So, this change has the `setHeader` stubbed, and sets the method to `GET`, after which `--computerForPath` works as expected.