Use defined 400 response object when request validation fails

Hi,

When setting up the `requestBody` validation, is there any way to return the defined 400 response object instead of the text/plain with the json validation info?
In the following example:

```
---
openapi: 3.0.0
info:
version: 1.0.0
title: Mock API
servers:
- url: http://localhost:19999
paths:
"/api/3.1/login/":
post:
description: Request access token
operationId: login using client id & secret
produces:
- "application/json"
requestBody:
description: Client id and secret
required: true
content:
application/json:
schema:
type: object
properties:
client_id:
description: The ID of the client
type: string
enum:
- my_client_id
client_secret:
description: Client secret
type: string
enum:
- very_secret_secret
required:
- client_id
- client_secret
additionalProperties: false
responses:
200:
description: "/login response"
content:
application/json:
example:
access_token: '1234567890'
token_type: doggie
expires_in: 1000
400:
description: "Bad Request. Invalid client ID/secret"
content:
application/json:
example:
message: "Blablabla, bad request"
documentation_url: "http://blabla.com"
```
I get the expected 200 OK with:
```
curl -X POST -H 'Content-Type:application/json' -H 'Accept-Type:application/json' -i http://localhost:19999/api/3.1/login/ \
-d '{"client_id":"my_client_id", "client_secret": "very_secret_secret"}'

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json
Date: Sun, 22 Dec 2019 21:58:25 GMT
Content-Length: 82

{
"access_token": "1234567890",
"expires_in": 1000,
"token_type": "doggie"
}%
```

But with a different `client_secret` other than the one defined in the schema `enum` I get:
```
curl -X POST -H 'Content-Type:application/json' -H 'Accept-Type:application/json' -i http://localhost:19999/api/3.1/login/ \
-d '{"client_id":"my_client_id", "client_secret": "invalid_secret"}'

HTTP/1.1 400 Bad Request
Access-Control-Allow-Origin: *
Date: Sun, 22 Dec 2019 21:58:47 GMT
Content-Length: 271
Content-Type: text/plain; charset=utf-8

Request body has an error: doesn't input the schema: Error at "/client_secret":JSON value is not one of the allowed values
Schema:
{
"description": "Client secret",
"enum": [
"very_secret_secret"
],
"type": "string"
}

Value:
"invalid_secret"
```

while what I would really like was the object I defined, i.e.:
```
{
"message": "Blablabla, bad request",
"documentation_url": "http://blabla.com"
}
```

Do you have any idea how I could achieve this? Thank you.

Reproduce the requestBody validation failure with the supplied OpenAPI document and curl commands, then trace how the mock server handles the 400 response versus validation errors. Done means an invalid client_secret returns the defined application/json 400 example instead of the text/plain validation report, while the valid request still returns 200.

Written by the indexing model from the issue text.