修复select表单placeholder设置不生效问题

src/Form/Field/Select.php

@@ -241,7 +241,7 @@ JS;
             'allowClear'  => true,
             'placeholder' => [
                 'id'   => '',
-                'text' => trans('admin.choose'),
+                'text' => $this->placeholder(),
             ],
         ], $this->config);
 
@@ -288,7 +288,7 @@ JS;
     {
         $configs = array_merge([
             'allowClear'         => true,
-            'placeholder'        => $this->label,
+            'placeholder'        => $this->placeholder(),
             'minimumInputLength' => 1,
         ], $this->config);
 
@@ -375,7 +375,7 @@ JS;
             'allowClear'  => true,
             'placeholder' => [
                 'id'   => '',
-                'text' => $this->label,
+                'text' => $this->placeholder(),
             ],
         ], $this->config);
 
@@ -403,6 +403,20 @@ JS;
         return parent::render();
     }
 
+    /**
+     * {@inheritDoc}
+     */
+    public function placeholder($placeholder = null)
+    {
+        if ($placeholder === null) {
+            return $this->placeholder ?: $this->label;
+        }
+
+        $this->placeholder = $placeholder;
+
+        return $this;
+    }
+
     /**
      * @return void
      */