Optional Header can be undefined

[jacopocarlini]

Issue

If in the openapi v3 there is an optional header for the request the generated client expects it as param (can be undefined)

Example openapi:

 "headers" : {
    "X-Request-Id" : {
      "description" : "This header identifies the call",
      "schema" : {
        "type" : "string"
      }
    }
  }

Expected:

I want to ignore the optional header. Now I'm forced to pass it to the client.