bug#25640: 26.0.50; eww default duckduckgo search results always in error

[José L. Doménech]

Entering keywords on eww always results in error. This
error aborts the rendering process although a render of the search
result page is displayed.

Example:
emacs -Q

M-x eww
emacs

Always a message with the following error is signaled:

error in process filter: eww-size-text-inputs: Wrong type argument: listp, 
((:method . "post") (:action . "/html/") (hidden :name "kl" :value "us-en"))
error in process filter: Wrong type argument: listp, ((:method . "post") 
(:action . "/html/") (hidden :name "kl" :value "us-en"))



In GNU Emacs 26.0.50.107 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9)
 of 2017-02-07 built on jose-E330
Repository revision: c939075b81b2b06c5ec040d7039fd20433509273
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:     Ubuntu 16.04.1 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Contacting host: duckduckgo.com:443
error in process filter: eww-size-text-inputs: Wrong type argument: listp, 
((:method . "post") (:action . "/html/") (hidden :name "kl" :value "us-en"))
error in process filter: Wrong type argument: listp, ((:method . "post") 
(:action . "/html/") (hidden :name "kl" :value "us-en"))
Making completion list...

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: es_ES.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Messages

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message dired dired-loaddefs rfc822 mml
mml-sec epa derived epg epg-config mm-decode mm-bodies mm-encode
mailabbrev gmm-utils mailheader sendmail network-stream starttls
url-http tls gnutls mail-parse rfc2231 url-gw nsm url-cache url-auth eww
puny mm-url gnus nnheader gnus-util rmail rmail-loaddefs rfc2047 rfc2045
ietf-drums mail-utils wid-edit mm-util mail-prsvr url-queue url
url-proxy url-privacy url-expand url-methods url-history url-cookie
url-domsuf url-util url-parse auth-source cl-seq eieio eieio-core
cl-macs eieio-loaddefs password-cache url-vars mailcap shr svg xml seq
byte-opt gv bytecomp byte-compile cl-extra help-mode easymenu cconv dom
subr-x browse-url format-spec cl-loaddefs pcase cl-lib time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript case-table
epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 171292 12410)
 (symbols 48 23647 1)
 (miscs 40 417 148)
 (strings 32 33455 6256)
 (string-bytes 1 1301140)
 (vectors 16 17866)
 (vector-slots 8 542670 8051)
 (floats 8 247 124)
 (intervals 56 2436 0)
 (buffers 976 13)
 (heap 1024 32851 9134))