[Dropdown] Apply btn-group class instead of style while using default tag.

Author: wxsms

docs/pages/components/Dropdown.md

@@ -23,7 +23,7 @@ Use `v-model` to indicate the dropdown status.
     <li><a role="button">Separated link</a></li>
   </template>
 </dropdown>
-<dropdown class="btn-group">
+<dropdown>
   <btn type="info">Split Button</btn>
   <btn type="info" class="dropdown-toggle"><span class="caret"></span></btn>
   <template slot="dropdown">

src/components/dropdown/Dropdown.vue

@@ -9,13 +9,10 @@
         this.tag,
         {
           class: {
+            'btn-group': this.tag === DEFAULT_TAG,
             dropdown: !this.dropup,
             dropup: this.dropup,
             open: this.show
-          },
-          style: {
-            'display': this.tag === DEFAULT_TAG ? 'inline-block' : null,
-            'vertical-align': this.tag === DEFAULT_TAG ? 'middle' : null
           }
         },
         [