Fix default json coder for datetime.

examples/main.py

@@ -1,3 +1,5 @@
+from datetime import date, datetime
+
 import aioredis
 import uvicorn
 from fastapi import FastAPI
@@ -31,6 +33,18 @@ async def clear():
     return await FastAPICache.clear(namespace="test")
 
 
+@app.get("/date")
+@cache(namespace="test", expire=20)
+async def get_data(request: Request, response: Response):
+    return date.today()
+
+
+@app.get("/datetime")
+@cache(namespace="test", expire=20)
+async def get_datetime(request: Request, response: Response):
+    return datetime.now()
+
+
 @app.on_event("startup")
 async def startup():
     redis = aioredis.from_url(url="redis://localhost")