[BUG] Description

Created by: neclimdul

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
What's the version of OpenAPI Generator used?
Have you search for related issues/PRs?
What's the actual output vs expected output?
[Optional] Bounty to sponsor the fix (example)

Description

Documentation for swagger generated code seems to be missing the content type.

openapi-generator version

3.0.0-latest

This is a regression from swagger-codegen-cli where it is generated correctly.

OpenAPI declaration file content or url

swagger: '2.0'
info:
  title: test
  version: 1.0.0
consumes:
  - application/json
produces:
  - application/json
paths: 
  /person:
    post:
      operationId: createPerson
      responses:
        200:
          description: "Success"
          schema:
            type: object

Command line used for generation

I didn't test all generators but I did check java and php and the problem existed in both so it seems to be an underlying problem.

docker run --rm -v ${PWD}:/local/ openapitools/openapi-generator-cli:latest generate \
    -i /local/test.yaml -g java -o /local/.build

Steps to reproduce

build the code using the above command
Inspect the generated markdown in the documentation folder.

Expected:

### HTTP request headers
 
  - **Content-Type**: application/json
  - **Accept**: application/json

Actual:

### HTTP request headers
 
  - **Content-Type**: Not defined
  - **Accept**: application/json

ambiguous set of steps to reproduce the bug.-->