Turn the headers of HTTPResponse from str to bytes automatically

[sanjiely]

Python: 3.6.4
Django: 2.0.3

When I return a HttpResponse object, Django library will do the job for me: turn my content from str to bytes silently.
But Django will not do the same work to the headers of HTTPResponse.

That is to say, the sample as below in the official document is not precisely correct.
(Page: ​https://docs.djangoproject.com/en/2.0/ref/request-response/)

>>> response = HttpResponse(my_data, content_type='application/vnd.ms-excel')
>>> response['Content-Disposition'] = 'attachment; filename="foo.xls"'

The precise usage should be:

>>> response['Content-Disposition'] = 'attachment; filename="foo.xls"'.encode('utf-8')

Otherwise, everthing will out of control If I have a Chinse character in the filename.
Wrong:

>>> response['Content-Disposition'] = 'attachment; filename="中中中中中foo.xls"'

Correct:

>>> response['Content-Disposition'] = 'attachment; filename=中中中中中foo.xls'.encode('utf-8')

Can we accept this ticket and do a minor improvement? Then we would happily only handle str in Django other than str and bytes together.
Thanks.

Yong Li