[BUG] Authorization Header; Case is not respected

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? Latest Master
• Have you search for related issues/PRs? Yes
• What's the actual output vs expected output? Header is accepted with a capitilsation
• [Optional] Bounty to sponsor the fix (example)

Description

An OAuth 2.0 Access Token is conveyed in a header called Authorization. This can be defined for example like below.
- description: 'An Authorisation Token as per https://tools.ietf.org/html/rfc6750'
explode: false
in: header
name: Authorization
required: true
schema:
type: string
style: simple

Unfortunately the capitalization requirements of the Authorization header isn't being respected by the generator

{
"error": [
{
"path": ".headers.authorization",
"message": "should have required property 'authorization'",
"errorCode": "required.openapi.validation"
}
]
}

So a request with Authorization results in an error being thrown.

openapi-generator version

Latest, it is not a regression.

OpenAPI declaration file content or url

        - description: 'An Authorisation Token as per https://tools.ietf.org/html/rfc6750'
          explode: false
          in: header
          name: Authorization
          required: true
          schema:
            type: string
          style: simple

Command line used for generation

openapi-generator generate -g nodejs-express-server -i account-info-openapi.yaml

Steps to reproduce

Generate the server, launch server, send a request with an Authorization header with a capital A.

Begin with the nodejs-express-server generator and the generated request validation path described by the reproduction. Generate from the supplied OpenAPI header definition, send Authorization with a capital A, and consider the issue done when the request is accepted without the lowercase-name required-property error.