Describe the bug
Putting the middleware on top of middleware list — as suggested by doc — doesn't allow next middlewares to potentially add/override HTTP headers through:

request.META['HTTP_CUSTOM_HEADER'] = "CUSTOM VALUE"

This is due to using django_guid.utils.get_id_from_header() in django_guid.middleware.process_incoming_request(), which triggers an earlier immutable HttpHeader object build, so any add/override headers intent has no impact.
Would be better to use request.META to get/set headers.

To Reproduce

1. Create a middleware

   def foo_middleware(get_response):
       def middleware(request):
           request.META["HTTP_X_FOO"] = "foo"
           return get_response(request)
   
       return middleware

2. Add middleware after django_guid

   MIDDLEWARE = [
       "django_guid.middleware.guid_middleware",
       "foo.foo_middleware",
   ]

3. Try to access the header in a view

   @api_view(["GET"])
   def test_view(request):
       assert request.headers.get("X-Foo") == "foo"

4. Now retry by removing "django_guid.middleware.guid_middleware" from MIDDLEWARE.