fix some return type declarations

src/Service.php

@@ -37,7 +37,7 @@ class Service extends Component implements ServiceInterface
     const ALC_BUCKET_OWNER_FULL_CONTROL = 'bucket-owner-full-control';
 
     /** @var string */
-    public $defaultBucket;
+    public $defaultBucket = '';
 
     /** @var string */
     public $defaultAcl = '';

src/base/commands/traits/Async.php

@@ -13,13 +13,11 @@ trait Async
     private $isAsync = false;
 
     /**
-     * @param bool $async
-     *
      * @return $this
      */
-    final public function async(bool $async = true)
+    final public function async()
     {
-        $this->isAsync = $async;
+        $this->isAsync = true;
 
         return $this;
     }

src/commands/ExistCommand.php

@@ -28,7 +28,7 @@ class ExistCommand extends ExecutableCommand implements HasBucket
      */
     public function getBucket(): string
     {
-        return $this->bucket;
+        return (string)$this->bucket;
     }
 
     /**
@@ -48,7 +48,7 @@ class ExistCommand extends ExecutableCommand implements HasBucket
      */
     public function getFilename(): string
     {
-        return $this->filename;
+        return (string)$this->filename;
     }
 
     /**

src/commands/GetUrlCommand.php

@@ -25,7 +25,7 @@ class GetUrlCommand extends ExecutableCommand implements HasBucket
      */
     public function getBucket(): string
     {
-        return $this->bucket;
+        return (string)$this->bucket;
     }
 
     /**
@@ -45,7 +45,7 @@ class GetUrlCommand extends ExecutableCommand implements HasBucket
      */
     public function getFilename(): string
     {
-        return $this->filename;
+        return (string)$this->filename;
     }
 
     /**

src/interfaces/commands/Asynchronous.php

@@ -10,9 +10,9 @@ namespace frostealth\yii2\aws\s3\interfaces\commands;
 interface Asynchronous
 {
     /**
-     * @param bool $async
+     * @return mixed
      */
-    public function async(bool $async = true);
+    public function async();
 
     /**
      * @return bool