### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

I Created an REST APIGatway with lambda handler using AWS CDK. when I hit the API the response header content type is always  set to "text/html"  instead of `application/json` . even if I changed the response template or added the header to lambda returned response. when I deploy the same stack on AWS account  the response header is fine and work as expected

### Expected Behavior

Response Header content-type to be `application/json`

### How are you starting LocalStack?

  `localstack start` command

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command`)

    `localstack start` command

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

I'm using AWS CDK typescript application and deploy it using `cdklocal` command on localstack

Using CDK follow the steps:
- create  REST ApiGateway
- create Lambda function 
- integrate lambda with APIGatway

On Command line:
- deploy using `cdklocal deploy` command

Finally
- test the endpoint and check the response header content-type




### Environment

```markdown
- OS: Windows 
- LocalStack: latest
```


### Anything else?

When I deploy the same stack on AWS account everything works fine and the response header content-type is `application/json`
status_code = 200
        if invocation_context.headers.get(HEADER_CONTENT_TYPE) == APPLICATION_JSON:
            try:
                mock_response = json.loads(request_payload)
                status_code = mock_response.get("statusCode", status_code)
            except Exception as e:
                LOG.warning("failed to deserialize request payload after transformation: %s", e)
                http_status = HTTPStatus(500)
                return MockIntegration._create_response(
                    http_status.value,
                    headers={"Content-Type": APPLICATION_JSON},
                    data=json.dumps({"message": f"{http_status.phrase}"}),
                )
        
        response = MockIntegration._create_response(
            status_code, invocation_context.headers, data=request_payload
        )
        response._content = self.response_templates.render(invocation_context, response=response)
        
        response = self.apply_response_parameters(invocation_context, response)
        if not invocation_context.headers.get(HEADER_CONTENT_TYPE):
            invocation_context.headers.update({HEADER_CONTENT_TYPE: APPLICATION_JSON})
        return response
class VelocityUtilApiGateway(VelocityUtil):
    """
    Simple class to mimic the behavior of variable '$util' in AWS API Gateway integration
    velocity templates.
    See: http://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html
    """
    def base64Encode(self, s):