self-contained: The content is in memory or obtained by
means that are independent from a connection or other entity.
Self-contained entities are generally IHttpEntity.IsRepeatable.
wrapping: The content is obtained from another entity.
This distinction is important for connection management with incoming
entities. For entities that are created by an application and only sent
using the HTTP components framework, the difference between streamed
and self-contained is of little importance. In that case, it is suggested
to consider non-repeatable entities as streamed, and those that are
repeatable (without a huge effort) as self-contained.