bug#22222: 24.4; eww ignores charset=utf-8 in text/plain

[Eli Zaretskii]

> From: trentbuck@gmail.com (Trent W. Buck)
> Date: Tue, 22 Dec 2015 15:11:56 +1100
> 
> In #emacs on Freenode, "snotglob" reported that
> 
>     When I open this doc with eww
>     
> https://raw.githubusercontent.com/howardabrams/pdx-emacs-hackers/master/workshops/keyboard-macros.org
>     it appears that some apostrophes are shown as \342\200\231
> 
> I can reproduce his fault.

It's not that EWW ignored the charset, but it did pretty strange
things with its value, and with decoding of the text in general.

I believe I fixed that in commit 9b0f182 on the emacs-25 branch.

Lars, I'd appreciate if you could eyeball the changeset and tell if I
broke some use case.  I couldn't wrap my head around the code there
that decoded stuff (like encoding with UTF-8 and then decoding with
something that must NOT be UTF-8 -- how can that do anything useful?),
so I just rewrote that as best I could.  If I broke something, please
show the broken use case, and I will look into it.

Thanks.