[Bug] Content-Type inconsistency between doc.produces and doc.response

Bug description

Using

@doc.produces({"status": str})

does not set (it actually does in the code, but sets it to None) content type on the route.

vs.

@doc.response(200, {"status": str})

This does automagically set the content type to application/json on the route.

Screenshots The screenshots of Swagger UI if this bug is related to it.

To Reproduce

from sanic.blueprints import Blueprint
from sanic.response import json
from sanic_openapi import doc

health_BP = Blueprint("health", url_prefix="/status")

@health_BP.route("/produces")
@doc.produces({"status": str}, description='Always returns {"test": "OK"}.')
async def status_produces(_req):
    return json({"status": "OK"})

@health_BP.route("/response")
@doc.response(200, {"test": str}, description='Always returns {"test": "OK"}.')
async def status_response(_req):
    return json({"test": "ok"})

Expected behavior Getting ~~automatic content-type~~... similar behaviour of the same/similar features/logic between these two doc.produces and doc.response(200, ...) - in this instance it's detection of Content-Type.

Basically I would expect that doc.produces is a shorthand for doc.responce(200, ...) but even looking under the hood, it seems to be somewhat different (why?).

Environment:

OS: Ubuntu 20.04
Browser: Mozilla Firefox 79.0b5
Version: sanic-openapi==0.6.1

swagger.json

```json { "basePath": null, "definitions": {}, "host": null, "info": { "version": "1.0.0", "title": "API", "description": "", "termsOfService": "", "contact": { "email": null }, "license": { "name": null, "url": null } }, "paths": { "/status/produces": { "get": { "operationId": "health.status_produces", "consumes": [ "application/json" ], "produces": [ null ], "tags": [ "health" ], "parameters": [], "responses": { "200": { "schema": { "type": "object", "properties": { "status": { "type": "string" } } }, "description": "Always returns {\"test\": \"OK\"}." } } } }, "/status/response": { "get": { "operationId": "health.status_response", "consumes": [ "application/json" ], "produces": [ "application/json" ], "tags": [ "health" ], "parameters": [], "responses": { "200": { "schema": { "type": "object", "properties": { "test": { "type": "string" } } }, "description": "Always returns {\"test\": \"OK\"}." } } } } }, "schemes": [ "http" ], "security": null, "securityDefinitions": null, "swagger": "2.0", "tags": [ { "name": "health" } ] } ```

sanic-org / sanic-openapi

[Bug] Content-Type inconsistency between doc.produces and doc.response #184