Expected ISE on forking/linking for sequential()/parallel() methods not always happen

[Dmitry Bessonov]

Hello,

Expected IllegalStateExceptions is not always thrown when stream (looks 
like) has been already linked to a child stream.

The problem occurs with sequential()/parallel() methods when source is 
of the same type:

         Stream s = Arrays.asList(1, 2, 3).parallelStream();
         Stream p1 = s.parallel();
         Stream p2 = s.parallel();
         Stream s1 = s.sequential();

No ISEs thrown. Just as nothing happens in the case of sequential stream:

         Stream s = Arrays.asList(1, 2, 3).stream();
         Stream f1 = s.filter( e -> true );
         Stream s1 = s.sequential();
         Stream s2 = s.sequential();
         Stream s3 = s.sequential();

s, s1, s2, s3 refer to the same instance.
The spec for sequential() gives some slight hint on this.
Method parallel() has no spec (in b69) at all.

I might assume that the verdict - if it's a bug or not depends on how 
exactly possibly thrown
"java.lang.IllegalStateException: Stream is already linked to a child 
stream"
is going to be specified.

-Dmitry