redfish: systems: eventlog: fix "Verify Subscribe An Event" test failed

Symptom:
"Verify Subscribe An Event" test item in test_event_service.robotgot failed.

Root cause:
From test report that error message is
Key HttpHeaders: [] != [{'Content': 'application/json'}]
The HttpHeaders content is not same as between ${payload} and ${resp.dict}
Thus, cause this test item got failed.

However, we found that bmcweb seems change the HttpHeaders response as NULL
in this commit "Nullify HttpHeaders per the specification".
Thus, the response "HttpHeaders": [] that didn't match payload Content in test item.

More detail can refer to below bmcweb commit:
Per the definition of HttpHeaders in the schema "This object shall be
null or an empty array in responses."  This commit does as the
specification commands.
https://github.com/openbmc/bmcweb/commit/ad22fefecaf7988fd7072dc71042efbf86fc5162

Solution:
Correct payload content to match bmcweb current design.

Tested:
Run robot "Verify Subscribe An Event" of test_event_service.robot

Signed-off-by: Tim Lee <timlee660101@gmail.com>
Change-Id: Id877bba7093508209ed2eb486ced775c8c26f1a7
diff --git a/redfish/systems/eventlog/test_event_service.robot b/redfish/systems/eventlog/test_event_service.robot
index bde18fb..ab25fdf 100644
--- a/redfish/systems/eventlog/test_event_service.robot
+++ b/redfish/systems/eventlog/test_event_service.robot
@@ -34,16 +34,13 @@
 
     Should Be Empty  ${subscription_list}
 
-    ${HttpHeaders_dict}=  Create Dictionary  Content=application/json
-    ${HttpHeaders_list}=  Create List  ${HttpHeaders_dict}
     ${RegistryPrefixes_list}=  Create List  Base  OpenBMC  TaskEvent
     ${ResourceTypes_list}=  Create List  Task
 
     ${payload}=  Create Dictionary
     ...  Context=Test_Context  Destination=https://${REMOTE_SERVER_IP}:${HTTPS_PORT}/
-    ...  EventFormatType=Event  Protocol=Redfish  HttpHeaders=${HttpHeaders_list}
-    ...  SubscriptionType=RedfishEvent  RegistryPrefixes=${RegistryPrefixes_list}
-    ...  ResourceTypes=${ResourceTypes_list}
+    ...  EventFormatType=Event  Protocol=Redfish  SubscriptionType=RedfishEvent
+    ...  RegistryPrefixes=${RegistryPrefixes_list}  ResourceTypes=${ResourceTypes_list}
 
     Redfish.Post  /redfish/v1/EventService/Subscriptions  body=&{payload}
     ...  valid_status_codes=[${HTTP_CREATED}]