ensure that graph attributes are not lost

- this entails making Module.isSealed==true if attributes are set
- also removed Module.nest(), which implied fixing replaceShape to form
  a CompositeModule where CopiedModule was used before (GraphModule and
  TlsModule)

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -132,7 +132,7 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
    * only to the contained processing stages).
    */
   override def withAttributes(attr: Attributes): Repr[Out] =
-    new Source(module.withAttributes(attr).nest())
+    new Source(module.withAttributes(attr))
 
   /**
    * Add the given attributes to this Source. Further calls to `withAttributes`