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".
Related to #31

```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.