with-output-to-string does not close its stream

I just thought to check if w-o-t-s is affected by the same problem as with-input-from-string (!197 (merged)) and it seems to be that this is the case:

CL-USER> (defvar stream-var)
STREAM-VAR
CL-USER> (with-output-to-string (local-stream-var)
           (setf stream-var local-stream-var))
""
CL-USER> (open-stream-p stream-var)
T

Expected: stream is closed, the last expression is NIL

I would suggest making the necessary changes and merging a regression test with the test for with-input-from-string, introduced in my MR above. If you agree I would like to go ahead and try to fix this.