async function support

[AmirEspahbodi]

Does this library also support async flask?
Routes that are not async work well with decorator validate
But those that are async have problems
error:
TypeError: The view function did not return a valid response. The return type must be a string, dict, list, tuple with headers or status, Response instance, or WSGI callable, but it was a coroutine

from flask import Blueprint
from .serializer import NotifySerializer
from flask_pydantic import validate, converters
notify_blueprint = Blueprint("notify", __name__)



@notify_blueprint.route("/notify", methods=["POST"])
@validate()
async def index(body: NotifySerializer):
     return body.count

[kouk]

@yctomwang i'm using the changes written by @MarekPikula here: #70
The changes are a bit out of date with the latest version of the package so I had to resolve the conflicts in my branch here:
https://github.com/kouk/flask-pydantic/tree/improve_typing