[BUG][GO] default response not handled for error cases

Created by: thiagoarrais

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?

Description

Current golang client does not handle default response data types when the the server returns an HTTP error code (4xx).

openapi-generator version

master

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  version: 1.0.0
  title: Swagger Petstore
  license:
    name: MIT
servers:
  - url: http://petstore.swagger.io/v1
paths:
  /pets:
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      responses:
        '201':
          description: Null response
        default:
          description: unexpected error
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/Error"
components:
  schemas:
    Error:
      required:
        - code
        - message
      properties:
        code:
          type: integer
          format: int32
        message:
          type: string

Command line used for generation

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i petstore.yaml -g go -o out

Steps to reproduce

1. Generate client as per command line above
2. Verify that generated code at api_pets.go checks HTTP status code against 0:

	if localVarHttpResponse.StatusCode >= 300 {
		newErr := GenericOpenAPIError{
			body:  localVarBody,
			error: localVarHttpResponse.Status,
		}
		if localVarHttpResponse.StatusCode == 0 {
			// ...               
		}
	}

That doesn't work because the status code will never be zero at that point.

Related issues/PRs

Couldn't find any.

Suggest a fix

Do not generate the check for status code when generating code for the default response.