Allow a Pathname to be passed as a Response body

This lets me use `Rails.root.join` like so:

```ruby
stub_request(:get, /example.com/).to_return(
  body: Rails.root.join('test/fixtures/foo.txt')
)
```

I didn't bother allowing a Pathname to be passed as a raw response as I've never used that functionality but I'm happy to also add that if you would like.