修复bug #9—第一次创建的时候，已经配置了text但是在调用[self refreshPlaceholderView];的时候，并没有…

…检查text是否有值.

WZBTextView-demo/WZBTextView/UITextView+WZB.m

@@ -282,6 +282,7 @@ - (void)refreshPlaceholderView {
         self.wzb_placeholderView.font = self.font;
         self.wzb_placeholderView.textAlignment = self.textAlignment;
         self.wzb_placeholderView.textContainerInset = self.textContainerInset;
+        self.wzb_placeholderView.hidden = (self.text.length > 0 && self.text);
     }
 }