System.Web.HttpResponse: fix default encoding

[deltragon]

Split out of #21247, as it is a separate simple change and also fixes a bug.

By default, .NET does not append the charset to the Content-Type header if the response is empty. This was corrected in mono in #866.
However, as soon as any content is written to the response, it should append the charset. When just using HttpResponse.Write(), this defaults to utf-8.
(Note: I reverse-engineered this behavior from the referencesource.)

This PR fixes this behavior, and adds an additional testcase for the non-empty response.
This testcase has not been run on windows/.NET, as I do not have mono set up there. Does someone else need to check if this test passes on .NET or is this checked as part of CI?

Fixes https://bugzilla.xamarin.com/23/23810/bug.html

[deltragon]

I am not sure if the CI failures are my fault? If they are, I don't understand the output properly.