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():
+            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)
+            if self.fileobj is None:
                 fh.close()

test/test_freeze.py

@@ -62,6 +62,15 @@ class FreezeTestCase(unittest.TestCase):
         finally:
             fh.close()
 
+    def test_memory_streams(self):
+        import io
+
+        for fmt in ('csv', 'json', 'tabson'):
+            with io.StringIO() as fd:
+                freeze(self.tbl.all(), format=fmt, fileobj=fd)
+                self.assertFalse(fd.closed, 'fileobj was closed for format %s' % fmt)
+                fd.getvalue() # should not throw
+
 
 class SerializerTestCase(unittest.TestCase):