bug#23253: 25.0.90; url-retrieve ignores url-mime-accept-string and most other url variables

[Clément Pit--Claudel]

The doc string of url-retrieve says:

> The variables ‘url-request-data’, ‘url-request-method’ and
> ‘url-request-extra-headers’ can be dynamically bound around the
> request; dynamic binding of other variables doesn’t necessarily
> take effect.

It seems that there is no way to set ‘url-mime-encoding-string’, 
‘url-mime-charset-string’, and ‘url-mime-accept-string’ for a single 
asynchronous query (things work with ‘url-retrieve-synchronously’, but not with 
‘url-retrieve’).

More precisely: this sends a query with ‘Accept: */*’, and prints nil:

    (let ((url-mime-accept-string "abc/def"))
      (url-retrieve "http://www.gnu.org/software/emacs/";
                    (lambda (&rest _)
                      (message "url-retrieve: %S"
                               url-mime-accept-string))))

but this sends a query with ‘Accept: abc/def’, and prints “abc/def”:

    (let ((url-mime-accept-string "abc/def"))
      (with-current-buffer
          (url-retrieve-synchronously
           "http://www.gnu.org/software/emacs/";)
        (message "url-retrieve: %S"
                 url-mime-accept-string)))

If this is exepcted, could we document the proper way to set an Accept header?

Clément.

In GNU Emacs 25.0.90.1 (x86_64-unknown-linux-gnu, GTK+ Version 3.10.8)
 of 2016-02-23 built on clem-w50-mint
Repository revision: f1dc3dc63ae4e454ca0e98eebbd80f0ba9d5c356
Windowing system distributor 'The X.Org Foundation', version 11.0.11501000
System Description:     Linux Mint 17.3 Rosa

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

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