Small simplification

src/ServerRequestFactory.php

@@ -199,18 +199,18 @@ public static function marshalHeaders(array $server)
         $headers = [];
         foreach ($server as $key => $value) {
             if ($value && strpos($key, 'HTTP_') === 0) {
-                $name = strtr(substr($key, 5), '_', ' ');
-                $name = strtr(ucwords(strtolower($name)), ' ', '-');
+                $name = substr($key, 5);
+                $name = strtr($name, '_', '-');
                 $name = strtolower($name);
 
                 $headers[$name] = $value;
                 continue;
             }
 
             if ($value && strpos($key, 'CONTENT_') === 0) {
-                $name = substr($key, 8); // Content-
-                $name = 'Content-' . (($name == 'MD5') ? $name : ucfirst(strtolower($name)));
+                $name = strtr($key, '_', '-');
                 $name = strtolower($name);
+
                 $headers[$name] = $value;
                 continue;
             }