ob-gnuplot: Fix stable file collision when converting data

* lisp/ob-gnuplot.el (org-babel-gnuplot-process-vars): Use cons cell
of all the code block parameters and the actual assigned variable
value to generate filename for storing the data.  This fixes a
scenario when code block has two or more variable assignments that
were wrongly saved into the same file.  Before the commit, only the
last assigned variable value has been used by gnuplot for _all_ the
variables.

lisp/ob-gnuplot.el

@@ -96,7 +96,7 @@ code."
 		 (if tablep val (mapcar 'list val)))
                ;; Make temporary file name stable with respect to data.
                ;; If we do not do it, :cache argument becomes useless.
-               (org-babel-temp-stable-file params "gnuplot-")
+               (org-babel-temp-stable-file (cons val params) "gnuplot-")
                params)
 	    (if (and (stringp val)
 		     (file-remote-p val)  ;; check if val is a remote file