Headers returned from function isn't lower cased

Headers returned from a mock function isn't lower cased, compare the two following examples.

``` javascript
scope.head('/').reply(200, '', { 'X-Test': 'Hello' })
```

``` javascript
scope.head('/').reply(() => [200, '', { 'X-Test': 'Hello' }])
```

The first one correctly has `res.headers['x-test']` set to "Hello", the other example on the other hand still has the key capitalised.
