outgoing bots: Fix header for generic servers.

For our bots that use GenericOutgoingWebhookService
(which are basically Zulip style bots), we now
include a "content-type" header of "application/json".

We accomplish this by having the service classes
implement their own custom method called
`send_data_to_server`. For the Slack-related
code, we just extracted code from `do_rest_call`,
and then for the Zulip-related code, we added
a `headers` parameter.

zerver/lib/outgoing_webhook.py

@@ -37,6 +37,13 @@ class GenericOutgoingWebhookService(OutgoingWebhookServiceInterface):
                         "trigger": event['trigger']}
         return json.dumps(request_data)
 
+    def send_data_to_server(self,
+                            base_url: str,
+                            request_data: Any) -> Response:
+        headers = {'content-type': 'application/json'}
+        response = requests.request('POST', base_url, data=request_data, headers=headers)
+        return response
+
     def process_success(self, response_json: Dict[str, Any],
                         event: Dict[str, Any]) -> Optional[Dict[str, Any]]:
         if "response_not_required" in response_json and response_json['response_not_required']:
@@ -78,6 +85,12 @@ class SlackOutgoingWebhookService(OutgoingWebhookServiceInterface):
 
         return request_data
 
+    def send_data_to_server(self,
+                            base_url: str,
+                            request_data: Any) -> Response:
+        response = requests.request('POST', base_url, data=request_data)
+        return response
+
     def process_success(self, response_json: Dict[str, Any],
                         event: Dict[str, Any]) -> Optional[Dict[str, Any]]:
         if "text" in response_json:
@@ -250,7 +263,10 @@ def do_rest_call(base_url: str,
                  event: Dict[str, Any],
                  service_handler: Any) -> None:
     try:
-        response = requests.request('POST', base_url, data=request_data)
+        response = service_handler.send_data_to_server(
+            base_url=base_url,
+            request_data=request_data,
+        )
         if str(response.status_code).startswith('2'):
             process_success_response(event, service_handler, response)
         else:

zerver/tests/test_outgoing_webhook_system.py

@@ -10,7 +10,12 @@ from django.test import override_settings
 from requests import Response
 from typing import Any, Dict, Tuple, Optional
 
-from zerver.lib.outgoing_webhook import do_rest_call, GenericOutgoingWebhookService
+from zerver.lib.outgoing_webhook import (
+    do_rest_call,
+    GenericOutgoingWebhookService,
+    SlackOutgoingWebhookService,
+)
+
 from zerver.lib.test_classes import ZulipTestCase
 from zerver.models import get_realm, get_user, UserProfile, get_display_recipient
 
@@ -59,6 +64,12 @@ class DoRestCallTests(ZulipTestCase):
             do_rest_call('', None, self.mock_event, service_handler)
             self.assertTrue(mock_send.called)
 
+        for service_class in [GenericOutgoingWebhookService, SlackOutgoingWebhookService]:
+            handler = service_class(None, None, None)
+            with mock.patch('requests.request', return_value=response):
+                do_rest_call('', None, self.mock_event, handler)
+                self.assertTrue(mock_send.called)
+
     def test_retry_request(self: mock.Mock) -> None:
         response = ResponseMock(500)