(fill-region, fill-region-as-paragraph):

If specified region ends before a newline, include that newline.

lisp/textmodes/fill.el

@@ -97,7 +97,12 @@ From program, pass args FROM, TO and JUSTIFY-FLAG."
 	    )))
 
     (save-restriction
-      (narrow-to-region from to)
+      (goto-char (max from to))
+      ;; If specified region ends before a newline,
+      ;; include that newline.
+      (if (and (eolp) (not (eobp)))
+	  (forward-char 1))
+      (narrow-to-region (min from to) (point))
       (goto-char (point-min))
       (skip-chars-forward "\n")
       (narrow-to-region (point) (point-max))
@@ -256,7 +261,12 @@ If `sentence-end-double-space' is non-nil, then period followed by one
 space does not end a sentence, so don't break a line there."
   (interactive "r\nP")
   (save-restriction
-   (narrow-to-region from to)
+    (goto-char (max from to))
+    ;; If specified region ends before a newline,
+    ;; include that newline.
+    (if (and (eolp) (not (eobp)))
+	(forward-char 1))
+    (narrow-to-region (min from to) (point))
     (goto-char (point-min))
     (while (not (eobp))
       (let ((initial (point))