made a small optimization

src/Symfony/Component/HttpFoundation/ServerBag.php

@@ -22,10 +22,9 @@ class ServerBag extends ParameterBag
     public function getHeaders()
     {
         $headers = array();
-
         foreach ($this->parameters as $key => $value) {
-            if ('http_' === strtolower(substr($key, 0, 5))) {
-                $headers[substr($key, 5)] = $value;
+            if ('HTTP_' === substr($key, 0, 5)) {
+                $headers[strtolower(substr($key, 5))] = $value;
             }
         }
 

src/Symfony/Component/HttpKernel/bootstrap.php

@@ -947,8 +947,8 @@ class ServerBag extends ParameterBag
     {
         $headers = array();
         foreach ($this->parameters as $key => $value) {
-            if ('http_' === strtolower(substr($key, 0, 5))) {
-                $headers[substr($key, 5)] = $value;
+            if ('HTTP_' === substr($key, 0, 5)) {
+                $headers[strtolower(substr($key, 5))] = $value;
             }
         }
         return $headers;

src/Symfony/Component/HttpKernel/bootstrap_cache.php

@@ -1173,8 +1173,8 @@ class ServerBag extends ParameterBag
     {
         $headers = array();
         foreach ($this->parameters as $key => $value) {
-            if ('http_' === strtolower(substr($key, 0, 5))) {
-                $headers[substr($key, 5)] = $value;
+            if ('HTTP_' === substr($key, 0, 5)) {
+                $headers[strtolower(substr($key, 5))] = $value;
             }
         }
         return $headers;