Button: avoid button splitting in button-group

packages/theme-default/src/button.css

@@ -151,6 +151,9 @@
     & .el-button {
       float: left;
       position: relative;
+      & + .el-button {
+        margin-left: 0;
+      }
 
       &:first-child {
         border-top-right-radius: 0;