Add content type to POST in endpoint_scaffold_acceptance_test

`Pliny::Helpers::Params` only looks at the body when the content type is `application/json`.
This PR is related to #31 

## Alternatives

```ruby
post "<%= url_path %>", MultiJson.encode({}), { "CONTENT_TYPE" => "application/json" }
```
would be also possible.

Or custom helpers like `post_json`, `patch_json`, `get_json` etc.