Add content-type headers in response in ASGI mode

sanic/asgi.py

@@ -328,6 +328,11 @@ class ASGIApp:
                 (b"content-length", str(len(response.body)).encode("latin-1"))
             ]
 
+        if "content-type" not in response.headers:
+            headers += [
+                (b"content-type", str(response.content_type).encode("latin-1"))
+            ]
+
         if response.cookies:
             cookies.update(
                 {

tests/test_asgi.py

@@ -9,7 +9,7 @@ from sanic import Sanic
 from sanic.asgi import MockTransport
 from sanic.exceptions import InvalidUsage
 from sanic.request import Request
-from sanic.response import text
+from sanic.response import json, text
 from sanic.websocket import WebSocketConnection
 
 
@@ -256,3 +256,27 @@ async def test_cookie_customization(app):
         assert cookie_map.get(k).get("value") == v.value
         if cookie_map.get(k).get("HttpOnly"):
             assert "HttpOnly" in v._rest.keys()
+
+
+@pytest.mark.asyncio
+async def test_json_content_type(app):
+    @app.get("/json")
+    def send_json(request):
+        return json({"foo": "bar"})
+
+    @app.get("/text")
+    def send_text(request):
+        return text("foobar")
+
+    @app.get("/custom")
+    def send_custom(request):
+        return text("foobar", content_type="somethingelse")
+
+    _, response = await app.asgi_client.get("/json")
+    assert response.headers.get("content-type") == "application/json"
+
+    _, response = await app.asgi_client.get("/text")
+    assert response.headers.get("content-type") == "text/plain; charset=utf-8"
+
+    _, response = await app.asgi_client.get("/custom")
+    assert response.headers.get("content-type") == "somethingelse"