JDK13: error: value mkString is not a member of java.util.stream.Stream[String]

## Reproduction Steps

using Scala 2.13.2 and Java 13.0.2 on MacOS,

```scala
scala> def foo(s: String) = s.lines.mkString(":")
error: value mkString is not a member of java.util.stream.Stream[String]
did you mean toString?
```

## Problem

There should be no error. 
