🐛 Fix jsonable_encoder for models with Config (#29)

but without json_encoders

tests/test_datetime_custom_encoder.py

@@ -0,0 +1,34 @@
+from datetime import datetime, timezone
+
+from fastapi import FastAPI
+from pydantic import BaseModel
+from starlette.testclient import TestClient
+
+
+class ModelWithDatetimeField(BaseModel):
+    dt_field: datetime
+
+    class Config:
+        json_encoders = {
+            datetime: lambda dt: dt.replace(
+                microsecond=0, tzinfo=timezone.utc
+            ).isoformat()
+        }
+
+
+app = FastAPI()
+model = ModelWithDatetimeField(dt_field=datetime(2019, 1, 1, 8))
+
+
+@app.get("/model", response_model=ModelWithDatetimeField)
+def get_model():
+    return model
+
+
+client = TestClient(app)
+
+
+def test_dt():
+    with client:
+        response = client.get("/model")
+    assert response.json() == {"dt_field": "2019-01-01T08:00:00+00:00"}