bug#17785: 24.4.50; EWW: Bales out during form submission

[Brian McKenna <brian@brianmckenna.org>]

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I got problems with the same chunk of code. My problem was that I was
using a form which didn't have a specific value, here's Twitter, for
example:

    <input type="hidden" name="scribe_log">

When trying to do a URL encode of that field, we'd try to construct a
pair but end up with:

    '(scribe_log)

Which would fail at encoding. It should encode to the same as
scribe_log having an empty value, that is "scribe_log=" and so should
be parsed as '(scribe_log "") - I've attached a patch which fixes
that.

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*** eww.el	2014-10-21 20:44:56.000000000 -0600
--- eww.el	2014-10-21 20:36:07.000000000 -0600
***************
*** 1001,1007 ****
        (when (and (consp elem)
  		 (eq (car elem) 'hidden))
  	(push (cons (plist-get (cdr elem) :name)
! 		    (plist-get (cdr elem) :value))
  	      values)))
      (if (and (stringp (cdr (assq :method form)))
  	     (equal (downcase (cdr (assq :method form))) "post"))
--- 1001,1007 ----
        (when (and (consp elem)
  		 (eq (car elem) 'hidden))
  	(push (cons (plist-get (cdr elem) :name)
! 		    (or (plist-get (cdr elem) :value) ""))
  	      values)))
      (if (and (stringp (cdr (assq :method form)))
  	     (equal (downcase (cdr (assq :method form))) "post"))