Refs #27025 -- Fixed a servers test on Python 3.6.

After https://hg.python.org/cpython/rev/4ea79767ff75/,
test_strips_underscore_headers fails with:
'Stub' object has no attribute 'sendall'.

tests/servers/test_basehttp.py

@@ -12,6 +12,9 @@ class Stub(object):
     def __init__(self, **kwargs):
         self.__dict__.update(kwargs)
 
+    def sendall(self, data):
+        self.makefile('wb').write(data)
+
 
 class WSGIRequestHandlerTestCase(SimpleTestCase):