ox: Fix priority bug in :title property

* lisp/ox.el (org-export--get-inbuffer-options): Return the empty
  string instead of nil when TITLE keywords has no value.
(org-export--get-buffer-attributes): Do not set :title property
early.
(org-export--get-global-options): Do not ignore anymore nil values.
Small refactoring.
(org-export-as): Correctly set :title here.

Thanks to Nicolas Richard for reporting it.
http://permalink.gmane.org/gmane.emacs.orgmode/87149

lisp/ox.el

@@ -1317,6 +1317,10 @@ The back-end could then be called with, for example:
 ;;   - category :: tree
 ;;   - type :: list of elements and objects
 ;;
+;; + `:input-buffer' :: Name of input buffer.
+;;   - category :: option
+;;   - type :: string
+;;
 ;; + `:input-file' :: Full path to input file, if any.
 ;;   - category :: option
 ;;   - type :: string or nil
@@ -1765,17 +1769,19 @@ Assume buffer is in Org mode.  Narrowing, if any, is ignored."
 	  (when (stringp value)
 	    (setq plist
 		  (plist-put plist property
-			     (org-element-parse-secondary-string
-			      value (org-element-restriction 'keyword))))))))))
+			     (or (org-element-parse-secondary-string
+				  value (org-element-restriction 'keyword))
+				 ;; When TITLE keyword sets an empty
+				 ;; string, make sure it doesn't
+				 ;; appear as nil in the plist.
+				 (and (eq property :title) ""))))))))))
 
 (defun org-export--get-buffer-attributes ()
   "Return properties related to buffer attributes, as a plist."
   ;; Store full path of input file name, or nil.  For internal use.
   (let ((visited-file (buffer-file-name (buffer-base-buffer))))
     (list :input-file visited-file
-	  :title (if (not visited-file) (buffer-name (buffer-base-buffer))
-		   (file-name-sans-extension
-		    (file-name-nondirectory visited-file))))))
+	  :input-buffer (buffer-name (buffer-base-buffer)))))
 
 (defun org-export--get-global-options (&optional backend)
   "Return global export options as a plist.
@@ -1788,23 +1794,22 @@ process."
 	(all (append (and backend (org-export-get-all-options backend))
 		     org-export-options-alist)))
     (dolist (cell all plist)
-      (let ((prop (car cell))
-	    (default-value (nth 3 cell)))
-	(unless (or (not default-value) (plist-member plist prop))
+      (let ((prop (car cell)))
+	(unless (plist-member plist prop)
 	  (setq plist
 		(plist-put
 		 plist
 		 prop
-		 ;; Eval default value provided.  If keyword is
+		 ;; Evaluate default value provided.  If keyword is
 		 ;; a member of `org-element-document-properties',
 		 ;; parse it as a secondary string before storing it.
 		 (let ((value (eval (nth 3 cell))))
-		   (if (not (stringp value)) value
-		     (let ((keyword (nth 1 cell)))
-		       (if (member keyword org-element-document-properties)
-			   (org-element-parse-secondary-string
-			    value (org-element-restriction 'keyword))
-			 value)))))))))))
+		   (if (and (stringp value)
+			    (member (nth 1 cell)
+				    org-element-document-properties))
+		       (org-element-parse-secondary-string
+			value (org-element-restriction 'keyword))
+		     value)))))))))
 
 (defun org-export--list-bound-variables ()
   "Return variables bound from BIND keywords in current buffer.
@@ -3017,6 +3022,14 @@ Return code as a string."
 	       (org-export-install-filters
 		(org-combine-plists
 		 info (org-export-get-environment backend subtreep ext-plist))))
+	 ;; Special case: provide original file name or buffer name as
+	 ;; default value for :title property.
+	 (unless (plist-get info :title)
+	   (plist-put
+	    info :title
+	    (let ((file (plist-get info :input-file)))
+	      (if file (file-name-sans-extension (file-name-nondirectory file))
+		(plist-get info :input-buffer)))))
 	 ;; Expand export-specific set of macros: {{{author}}},
 	 ;; {{{date}}}, {{{email}}} and {{{title}}}.  It must be done
 	 ;; once regular macros have been expanded, since document