fix(components): button displays icon only (#4019)

* fix(components): button displays icon only

* docs: add chinese button

docs/examples/button/basic.vue

@@ -6,6 +6,7 @@
     <el-button type="info">Info</el-button>
     <el-button type="warning">Warning</el-button>
     <el-button type="danger">Danger</el-button>
+    <el-button>中文</el-button>
   </el-row>
 
   <el-row>

packages/components/button/src/button.vue

@@ -18,9 +18,9 @@
     @click="handleClick"
   >
     <i v-if="loading" class="el-icon-loading"></i>
-    <i v-if="icon && !loading" :class="icon"></i>
+    <i v-else-if="icon" :class="icon"></i>
     <span
-      v-else-if="$slots.default"
+      v-if="$slots.default"
       :class="{ 'el-button__text--expand': shouldAddSpace }"
     >
       <slot></slot>