[BUG] Header duplicate on subdependency

Describe the bug Getting duplicate headers on API methods with subdependencies requiring the same dependency with a Header argument.

To Reproduce I’ve created an example repo to reproduce https://github.com/henriklindgren/fastapi-header-bug Steps to reproduce the behavior:

1. Create an API-method which uses the same dependency, with a Header argument, in each of 2 paths of the dependency tree. Like this https://github.com/henriklindgren/fastapi-header-bug/blob/master/app/main.py
2. Visit generated OpenApi json and see duplicate header requirements for the method.

"parameters": [
  {
    "required": true,
    "schema": {
      "title": "Someheader",
      "type": "string"
    },
    "name": "someheader",
    "in": "header"
  },
  {
    "required": true,
    "schema": {
      "title": "Someheader",
      "type": "string"
    },
    "name": "someheader",
    "in": "header"
  }
]

Expected behavior A single header requirement shown in the OpenApi definition.

Environment:

• OS: Linux
• FastAPI Version: 0.33.0
• Python Version: 3.7.3

[dmig-alarstudios]

@tiangolo could you please have a look at this?

[dmig-alarstudios]

I think the fix would be:

- response.headers.raw.extend(sub_response.headers.raw)
+ if response is not sub_response:
+   response.headers.raw.extend(sub_response.headers.raw)