Update Accept.php (#40)

Fixes: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated

src/Http/Parser/Accept.php

@@ -67,7 +67,7 @@ public function parse(Request $request, $strict = false)
     {
         $pattern = '/application\/'.$this->standardsTree.'\.('.$this->subtype.')\.([\w\d\.\-]+)\+([\w]+)/';
 
-        if (! preg_match($pattern, $request->header('accept'), $matches)) {
+        if (! preg_match($pattern, $request->header('accept', ''), $matches)) {
             if ($strict) {
                 throw new BadRequestHttpException('Accept header could not be properly parsed because of a strict matching process.');
             }