ob-gnuplot: Use org backend when assigning table to variable

* lisp/ob-gnuplot.el (org-babel-gnuplot-table-to-data): Switch to 'org
backend when exporting table data to plain text table.  ascii backend
incorrectly handles quote transcoders that change the column width and
sometimes throws an error.  (ascii backend simply ignores transcoded
table cells and calculates width based on the initial cell text).

lisp/ob-gnuplot.el

@@ -287,11 +287,21 @@ Pass PARAMS through to `orgtbl-to-generic' when exporting TABLE."
   (with-temp-file data-file
     (insert (let ((org-babel-gnuplot-timestamp-fmt
 		   (or (plist-get params :timefmt) "%Y-%m-%d-%H:%M:%S")))
-	      (orgtbl-to-generic
-	       table
-	       (org-combine-plists
-		'(:sep "\t" :fmt org-babel-gnuplot-quote-tsv-field :raw t :backend ascii)
-		params)))))
+              (replace-regexp-in-string
+               ;; org export backend adds "|" at the beginning/end of
+               ;; the table lines.  Strip those.
+               "^|\\(.+\\)|$"
+               "\\1"
+	       (orgtbl-to-generic
+	        table
+	        (org-combine-plists
+		 '( :sep "\t" :fmt org-babel-gnuplot-quote-tsv-field
+                    ;; Two setting below are needed to make :fmt work.
+                    :raw t
+                    ;; Use `org', not `ascii' because `ascii' may
+                    ;; sometimes mishandle quoted strings.
+                    :backend org)
+		 params))))))
   data-file)
 
 (provide 'ob-gnuplot)