make invoice->uuid() method fluent.

Author: m-ostadi

src/Invoice.php

@@ -59,6 +59,7 @@ public function uuid($uuid = null)
         }
 
         $this->uuid = $uuid;
+        return $this;
     }
 
     /**
@@ -68,7 +69,7 @@ public function uuid($uuid = null)
      */
     public function getUuid($needNumericUuid = false)
     {
-        if($needNumericUuid){
+        if ($needNumericUuid) {
             $this->uuid = !is_numeric($this->uuid) ? crc32($this->uuid) : $this->uuid;
         }
         return $this->uuid;