Fix bug (reported by Oxygen) where using the API to write a document …

…resulted in an attempt to write to a closed handle

src/com/xmlcalabash/io/WritableDocument.java

@@ -177,7 +177,6 @@ public void store(OutputStream ostream)
                 	}
                     runtime.error(ex);
                 }
-                S9apiUtils.serialize(runtime, doc, serializer);
             }