Make service API client do partial updates

The service API client was updating every attribute of a service. Which,
while kinda clunky, is fine…

…until something calling it doesn’t pass in every attribute of the
current service. It was then defaulting optional parameters to `None`.
Which resulted in a bug whereby every time a service was set to live,
its `reply_to_address` and `sms_sender_name` got overwritten to be
empty.

This commit changes the `update` method to only require the service ID,
and pass whatever other named arguments it received straight through to
the API. The API handles partial updates just fine (I think).

tests/conftest.py

@@ -125,18 +125,20 @@ def mock_create_service(mocker):
 
 @pytest.fixture(scope='function')
 def mock_update_service(mocker):
-    def _update(service_id,
-                service_name,
-                active,
-                message_limit,
-                restricted,
-                users,
-                email_from,
-                reply_to_email_address=None,
-                sms_sender=None):
+    def _update(service_id, **kwargs):
         service = service_json(
-            service_id, service_name, users, message_limit=message_limit,
-            active=active, restricted=restricted, email_from=email_from, reply_to_email_address=reply_to_email_address)
+            service_id,
+            **{key: kwargs.get(key) for key in [
+                'name',
+                'users',
+                'message_limit',
+                'active',
+                'restricted',
+                'email_from',
+                'reply_to_email_address',
+                'sms_sender'
+            ]}
+        )
         return {'data': service}
 
     return mocker.patch(
@@ -145,14 +147,11 @@ def mock_update_service(mocker):
 
 @pytest.fixture(scope='function')
 def mock_update_service_raise_httperror_duplicate_name(mocker):
-    def _update(service_id,
-                service_name,
-                active,
-                limit,
-                restricted,
-                users,
-                email_from):
-        json_mock = Mock(return_value={'message': {'name': ["Duplicate service name '{}'".format(service_name)]}})
+    def _update(
+        service_id,
+        **kwargs
+    ):
+        json_mock = Mock(return_value={'message': {'name': ["Duplicate service name '{}'".format(kwargs.get('name'))]}})
         resp_mock = Mock(status_code=400, json=json_mock)
         http_error = HTTPError(response=resp_mock, message="Default message")
         raise http_error