force initial page load response to html

Fix Firefox setting cookies before navigating.

Addresses https://github.com/groupon-testium/testium/issues/71
This function is pretty confusing (I had to look twice to understand that this is only used once and just statically writes a response). Could we change it to something like this:

```
emptySuccess = (response) ->
  response.statusCode = 200
  response.setHeader 'Content-Type', 'text/html'
  response.end()
```

Or, alternatively:

```
emptySuccess = (response) ->
  response.writeHead 200,  { 'Content-Type': 'text/html' }
  response.end()
```

Setting the status code as a property and then just explicitly passing it into writeHead in the next line seems a little weird.