Make erc completion case-insensitive again

* lisp/erc/erc.el (erc-completion-at-point): Make erc completion
case-insensitive again (bug#11360).

etc/NEWS

@@ -428,6 +428,10 @@ specified message types for the respective specified targets.
 ---
 *** Reconnection is now asynchronous.
 
+---
+*** Nick completion is now case-insentive again after inadvertently
+being made case-sensitive in Emacs 24.2.
+
 ** Midnight-mode
 
 ---

lisp/erc/erc.el

@@ -1141,7 +1141,7 @@ which the local user typed."
     (define-key map "\C-c\C-u" 'erc-kill-input)
     (define-key map "\C-c\C-x" 'erc-quit-server)
     (define-key map "\M-\t" 'ispell-complete-word)
-    (define-key map "\t" 'completion-at-point)
+    (define-key map "\t" 'erc-completion-at-point)
 
     ;; Suppress `font-lock-fontify-block' key binding since it
     ;; destroys face properties.
@@ -3991,6 +3991,13 @@ Prompt for one if called interactively."
                          (format "MODE %s +k %s" tgt key)
                        (format "MODE %s -k" tgt)))))
 
+(defun erc-completion-at-point ()
+  "Perform complection on the text around point case-insentitively.
+See `completion-at-point'."
+  (interactive)
+  (let ((completion-ignore-case t))
+    (completion-at-point)))
+
 (defun erc-quit-server (reason)
   "Disconnect from current server after prompting for REASON.
 `erc-quit-reason' works with this just like with `erc-cmd-QUIT'."