Fix decoding of text in URLs retrieved by EWW

Fix decoding of text in URLs retrieved by EWW

* lisp/net/eww.el (eww-render): Pass 'charset' to
'eww-display-raw'.  Use the value of 'last-coding-system-used', if
non-nil, to set 'buffer-file-coding-system' of the buffer where we
show the URL.
(eww-display-html, eww-display-raw): Decode the text correctly,
using the charset found in the headers, and defaulting to UTF-8.
If the user told us to use a specific encoding, override the
charset from the headers.  (Bug#22222)