Fix empty SOAPAction header

SOAPAction header should never be "None".

When `self.operation.soapaction` is `None` it was stringifyed into `"None"` when it should be `""`.

src/zeep/wsdl/messages/soap.py

@@ -84,7 +84,11 @@ def serialize(self, *args, **kwargs):
         # XXX: This is only used in Soap 1.1 so should be moved to the the
         # Soap11Binding._set_http_headers(). But let's keep it like this for
         # now.
-        headers = {"SOAPAction": '"%s"' % self.operation.soapaction}
+        headers = {
+            "SOAPAction": '"%s"' % self.operation.soapaction
+            if self.operation.soapaction
+            else '""'
+        }
         return SerializedMessage(path=None, headers=headers, content=envelope)
 
     def deserialize(self, envelope):