+hco 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 couple of 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.

Cc @sirthias & @jrudolph.