=act #21774 fix ByteString.toString to use fixed stringPrefix for all subclasses (#21775)

Otherwise, with 2.12,

ByteString().toString == "ByteString.ByteString1C()"

which would expose implementation details in the string representation.
This can lead to failing tests due to test expecting a particular string
representation of a ByteString which might be bad practice, yes, but is also
convenient.

The change is due to a fix in Scala for SI-9019 for which the string representation
of TraversableLike was changed which ByteString inherits.
See https://github.com/scala/scala/pull/5258/files

akka-actor-tests/src/test/scala/akka/util/ByteStringSpec.scala

@@ -600,6 +600,12 @@ class ByteStringSpec extends WordSpec with Matchers with Checkers {
           a.asByteBuffers.zip(a.getByteBuffers().asScala).forall(x ⇒ x._1 == x._2)
         }
       }
+
+      "toString should start with ByteString(" in {
+        check { (bs: ByteString) ⇒
+          bs.toString.startsWith("ByteString(")
+        }
+      }
     }
     "behave like a Vector" when {
       "concatenating" in { check { (a: ByteString, b: ByteString) ⇒ likeVectors(a, b) { _ ++ _ } } }