Try to remove HttpResponseWrapper

[sfisol]

In actix-web 4 HttpResponse is no longer implementing Future (actix/actix-web#2601) which renders a problem in paperclip while inserting closure as a handler, for example:

web::to(|| async { HttpResponse::Ok().finish() })

A HttpResponseWrapper was introduced to workaround this problem but it would be better to refactor paperclips' impls and remove the wrapper.

More information in this thread:
#314 (comment)