TreeMap addAll should retain existing keys

[lrytz]

After scala/scala#9291

scala> scala.collection.immutable.TreeMap[String, String]()(_ compareToIgnoreCase _) + ("a" -> "1") + ("A" -> "2")
val res0: scala.collection.immutable.TreeMap[String,String] = TreeMap(a -> 2)

scala> scala.collection.immutable.TreeMap[String, String]()(_ compareToIgnoreCase _) ++ Seq("a" -> "1", "A" -> "2")
val res1: scala.collection.immutable.TreeMap[String,String] = TreeMap(A -> 2)

res1 is wrong