[FIX] core: converse existing context in JSON-RPC

Define the following controller:

    @route('/get-context', auth='public', type='json', website=True):
        return request.env.context

Access that route via JSON-RPC providing a context:

    this._rpc({
        route: '/get-context',
        params: {
            context: {"a": 1}
        }
    })

Since httpocalypse, it replaces the context entirely instead of updating
it. i.e. before you would get along those lines:

    {"a": 1, "lang": "en_US", "tz": "Europe/Brussels", "uid": 2}

but since you get instead (and that's wrong):

    {"a": 1}

Note that after this commit, this merging-context behavior will be the
same whether or not website=True is defined (while it was only there for
website=True before).

Discovered via opw-2885948

X-original-commit: 7887cfc3f177d9e64f008aabb8bd710f54024dea
Part-of: odoo/odoo#107179

odoo/addons/test_http/tests/test_echo_reply.py

@@ -170,4 +170,6 @@ class TestHttpEchoReplyJsonWithDB(TestHttpBase):
         })
         res = self.db_url_open("/test_http/echo-json-context", data=payload, headers=CT_JSON)
         self.assertEqual(res.status_code, 200)
-        self.assertEqual(res.text, '{"jsonrpc": "2.0", "id": 0, "result": {"name": "Thor"}}')
+        self.assertEqual(res.text, '{"jsonrpc": "2.0", "id": 0, "result": '
+            f'{{"lang": "en_US", "tz": false, "uid": {self.jackoneill.id}, "name": "Thor"}}'
+            '}')

odoo/http.py

@@ -1798,7 +1798,7 @@ class JsonRPCDispatcher(Dispatcher):
         self.request.params = dict(self.jsonrequest.get('params', {}), **args)
         ctx = self.request.params.pop('context', None)
         if ctx is not None and self.request.db:
-            self.request.update_env(context=ctx)
+            self.request.update_context(**ctx)
 
         if self.request.db:
             result = self.request.registry['ir.http']._dispatch(endpoint)