* rails 5.0.0.1
* rspec 3.5

I have inherited a code base. I am busy writing integration tests to tie down the app functionality before I consider refactoring. 

I have the following lines in a controller concern `before_action`. It seems to read the request body. The `json` value here is used to extract an identifier used to authenticate the request. 

```
request.body.rewind
body = request.body.read
json = JSON.parse(body) unless body.empty?

```

I need to test that the authentication happens correctly. 
How can I set the `request.body` for a GET request spec?