dataset.freeze should not close provided fileobj

Test and solution for issue pudo/dataset#147

dataset/freeze/format/fcsv.py

@@ -47,4 +47,5 @@ class CSVSerializer(Serializer):
 
     def close(self):
         for writer, fh in self.handles.values():
-            fh.close()
+            if fh != self.fileobj:
+                fh.close()

dataset/freeze/format/fjson.py

@@ -55,4 +55,5 @@ class JSONSerializer(Serializer):
                                           self.export.get('callback'),
                                           data)
             fh.write(data)
-            fh.close()
+            if self.fileobj is None:
+                fh.close()