bug#17958: eww-submit mishandles the POST method, no action forms

[Ivan Shmakov <ivan@siamics.net>]

Package:  emacs

	As evidenced with the form at [1], EWW currently (as of
	36634f669f2c) mishandles the case where an HTML form uses
	‘method="POST"’ but specifies no ‘action’ attribute.  Namely,
	instead of interpreting missing ‘action’ as meaning “this very
	same URI”, EWW uses the URI with the ‘path’ component discarded.

	Granted, missing ‘action’ is special-cased for GET forms:

  1034		(if (cdr (assq :action form))
  1035		    (shr-expand-url (cdr (assq :action form))
  1036				    eww-current-url)
  1037		  eww-current-url)

	While POST forms get no such treatment:

  1030		  (eww-browse-url (shr-expand-url (cdr (assq :action form))
  1031						  eww-current-url)))

	However, I believe that the real culprit is shr-expand-url,
	which mishandles the nil ‘uri’ case:

(mapcar (lambda (x) (shr-expand-url x "http://example.com/welcome/"))
        '("hello" "/world" nil))
;; ⇒
("http://example.com/welcome/hello"
 "http://example.com/world"
 "http://example.com")

	My expectation for the last result would be the ‘base’ argument
	unchanged (i. e., http://example.com/welcome/.)

	Thus, I suggest changing shr-expand-url to return not the 0th
	element of the (parsed) ‘base’ (see below), but the 3rd.

   596	  (cond ((or (not url)
   597		     (not base)
   598		     (string-match "\\`[a-z]*:" url))
   599		 ;; Absolute URL.
   600		 (or url (car base)))

[1] https://tools.wmflabs.org/guc/?user=2001:db8:1337::cafe

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