fix(pull-down-refresh): t-class-loading configuration does not take effect (#3896)

Author: betavs

packages/components/pull-down-refresh/pull-down-refresh.wxml

@@ -50,6 +50,7 @@
         size="{{loadingProps.size || '50rpx'}}"
         text="{{loadingProps.text || loadingTexts[refreshStatus]}}"
         theme="{{loadingProps.theme || 'circular'}}"
+        t-class="{{prefix}}-class-loading"
         t-class-indicator="{{prefix}}-class-indicator"
       />
       <view wx:elif="{{refreshStatus >= 0}}" class="{{classPrefix}}__text {{prefix}}-class-text"