Make `Content-Length` header public again and add integration tests

This reverts the change made for #15800. Both `Content-Length` and
`Content-Type` headers are needed by Play and may be needed by other
libraries that integrate closely with akka-http-code. Added a few
integration tests outside the akka package to ensure that objects
needed by external libraries will remain visible. We may wish to add
more integration tests in the future, but theses tests should capture
the basic functionality that is needed by external libraries.