修复 发送图片类型的消息，图片传输完毕后会dom 会重绘的问题

components/uni-im-msg-list/uni-im-msg-list.vue

 <template>
   <view class="root">
     <uni-im-list v-if="visibleMsgList.length" class="uni-im-list" :scrollTop="scrollTop" :scroll-into-view="scrollIntoView" :paddingBottom="paddingBottom" ref="uni-im-list" @scroll="onScroll" @scrolltolower="onScrollToLower">
-     <template v-for="(msg,index) in visibleMsgList" :key="msg._id || msg.unique_id">
+     <template v-for="(msg,index) in visibleMsgList" :key="msg.unique_id || msg._id">
 		 <uni-im-list-item :ref="'item-'+index">
 		   <view v-if="index === 0" class="data-state-tip-box" @appear="beforeLoadMore">
        <uni-im-load-state v-if="canAnchor || !isMpPlatform" :status="hasMore?'loading':'noMore'"