Fix the __exit__ clause

Not the best fix, but it works

Author: mvantellingen

src/zeep/client.py

@@ -218,7 +218,8 @@ def __enter__(self):
         return self
 
     def __exit__(self, exc_type=None, exc_value=None, traceback=None):
-        self.transport.close()
+        if hasattr(self.transport, "close"):
+            self.transport.close()
 
 
 class AsyncClient(Client):