Android clang complains about the fact it cannot move the
`std::unique_ptr<ContentResponse>` into a `std::unique_ptr<Response>&&`
(for the implicit `std::unique_ptr<Response>` constructor).
Let's help him a bit.
This is only an "interface" for now as other type of response (entry) may
be "transformed" to a ContentResponse.
We cannot move all the code in the class.