make pretty-print-newline work on any output port

The documentation says that it should work on any output port,
although there's special treatment of ports that originate
from `pretty-print` itself.

Closes #1579.

pkgs/racket-test-core/tests/racket/pretty.rktl

@@ -525,6 +525,13 @@
     (pp v))
   (test "(a 1)\n" get-output-string o))
 
+;; ----------------------------------------
+;; check that `pretty-print-newline` works on any output port
+
+(let ([o (open-output-bytes)])
+  (pretty-print-newline o 17)
+  (test "\n" get-output-string o))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/collects/racket/pretty.rkt

@@ -373,8 +373,14 @@
 (define orig-write (port-write-handler (open-output-string)))
 
 (define (pretty-print-newline pport width)
-  (let-values ([(l col p) (port-next-location pport)])
-    ((printing-port-print-line pport) #t (or col 0) width)))
+  (cond
+   [(printing-port? pport)
+    (let-values ([(l col p) (port-next-location pport)])
+      ((printing-port-print-line pport) #t (or col 0) width))]
+   [(output-port? pport)
+    (newline pport)]
+   [else
+    (raise-argument-error 'pretty-print-newline "output-port?" pport)]))
 
 (define (tentative-pretty-print-port-transfer a-pport pport)
   (let ([content ((get a-pport print-port-info-get-content))])