Fix unit tests for Python 3.6 (#431)

Python 3.6 complains about 'AttributeError: 'MockRequest' object has no attribute 'sendall'

tests/test_api.py

@@ -101,6 +101,8 @@ class MockRequest(object):
     def makefile(self, *args, **kwargs):
         return IO(self.request)
 
+    def sendall(self, *args, **kwargs):
+        pass
 
 class MockRestApiServer(RestApiServer):