AsyncHttpClient loses context after StreamedAsyncHandler

[milanvdm]

Describe the bug

The trace is incorrectly propagated when using a StreamedAsyncHandler.

Steps to reproduce

The test setup uses Scala and depends on the latest master branch of ElasticAPM

1. Checkout https://github.com/milanvdm/scala-elastic-apm/blob/check-edge-case
2. Start the docker-compose
3. Run the application: sbt reStart or sbt run
4. It will lose the context at: https://github.com/milanvdm/scala-elastic-apm/blob/check-edge-case/src/main/scala/me/milan/main/future/DummyHttpBackend.scala#L76