Wow I feel stupid here (facepalm) -- I was setting `let(:raw_data) { params.to_json }` instead of `let(:raw_post) { ....}`. I was getting this error doing the following:

```
post '/users' do
  let(:raw_data) { params.to_json }
  
  example 'creating a user' do
    do_request raw_data
  end
end
```

This causes the error where your passing do_request a string and you get an error saying inject method is not defined (which is accurate).