In #36996, the behavior of `content_type=` has been changed to treat a
`content_type` itself as a mime type if a `content_type` doesn't contain
`charset=...` part.

i.e.

```ruby
response.content_type = "text/html; fragment"
response.media_type # => "text/html; fragment"

response.content_type = "text/html; fragment; charset=utf-16"
response.media_type # => "text/html"
```

That is tricky and strange.

I think that we cannot distinguish whether extra part is a part of mime
type or not for now. So at least in Rails 6.0, we should not discard
extra part conservatively as before.