Added 7.4 to travis.yml.

Fixed a bug where not all request headers were available when running under apache server.

.travis.yml

@@ -2,6 +2,7 @@ language: php
 php:
   - "7.2"
   - "7.3"
+  - "7.4"
 
 before_script:
   - composer selfupdate

src/Message/ServerRequest.php

@@ -479,6 +479,7 @@ class ServerRequest extends Request implements ServerRequestInterface
      */
     protected function getServerRequestHeaders()
     {
+        if (!function_exists('apache_get_version'))  {
             // http://www.php.net/manual/en/function.getallheaders.php#84262
             $headers = array();
             foreach ($_SERVER as $name => $value) {
@@ -488,6 +489,10 @@ class ServerRequest extends Request implements ServerRequestInterface
             }
             return $headers;
         }
+        else {
+            return apache_request_headers();
+        }
+    }
 
     /**
      * @param array $root