Params by plug convention should contain merged path, body and query parameters

[hamir-suspect]

According to Plug documentation conn.params is

the request params, the result of merging the :path_params on top of :body_params on top of :query_params

When using OpenApiSpex.Plug.CastAndValidate plug the params will overwrite only with params from :path | :query | :header | :cookie meaning that there is never parameters from request_body there.

This should be clearly stated in CastAndValidate plug like it is for cast plug as it can cause some confusion.

[mbuhot]

The library was initially implemented with all params merged into conn.params as per the docs. Unfortunately it is problematic with struct based body schemas.

If your body schema is a struct, the adding additional query params creates a struct with undeclared fields set. It technically works since structs are just maps, but it is confusing and may cause type errors in future elixir versions.