Log a warning when headers have been sent.

When headers have already been sent, we can be more helpful by logging
a warning. I've chosen warning as generally production systems I've
worked on have WARN+ enabled as those severity levels indicate issues
that need humans to fix them.

Refs #9159

src/Network/Response.php

@@ -16,6 +16,7 @@
 
 use Cake\Core\Configure;
 use Cake\Filesystem\File;
+use Cake\Log\Log;
 use Cake\Network\Exception\NotFoundException;
 use DateTime;
 use DateTimeZone;
@@ -446,7 +447,10 @@ public function send()
      */
     public function sendHeaders()
     {
-        if (headers_sent()) {
+        $file = $line = null;
+        if (headers_sent($file, $line)) {
+            Log::warning("Headers already sent in {$file}:{$line}");
+
             return;
         }