fixes #4156: don't override border radius if stacked nav has one child

docs/assets/css/bootstrap.css

@@ -3686,15 +3686,21 @@ input[type="submit"].btn.btn-mini {
 }
 
 .nav-tabs.nav-stacked > li:first-child > a {
-  -webkit-border-radius: 4px 4px 0 0;
-     -moz-border-radius: 4px 4px 0 0;
-          border-radius: 4px 4px 0 0;
+  -webkit-border-top-right-radius: 4px;
+          border-top-right-radius: 4px;
+  -webkit-border-top-left-radius: 4px;
+          border-top-left-radius: 4px;
+  -moz-border-radius-topright: 4px;
+  -moz-border-radius-topleft: 4px;
 }
 
 .nav-tabs.nav-stacked > li:last-child > a {
-  -webkit-border-radius: 0 0 4px 4px;
-     -moz-border-radius: 0 0 4px 4px;
-          border-radius: 0 0 4px 4px;
+  -webkit-border-bottom-right-radius: 4px;
+          border-bottom-right-radius: 4px;
+  -webkit-border-bottom-left-radius: 4px;
+          border-bottom-left-radius: 4px;
+  -moz-border-radius-bottomright: 4px;
+  -moz-border-radius-bottomleft: 4px;
 }
 
 .nav-tabs.nav-stacked > li > a:hover {

less/navs.less

@@ -171,10 +171,10 @@
   .border-radius(0);
 }
 .nav-tabs.nav-stacked > li:first-child > a {
-  .border-radius(4px 4px 0 0);
+  .border-top-radius(4px);
 }
 .nav-tabs.nav-stacked > li:last-child > a {
-  .border-radius(0 0 4px 4px);
+  .border-bottom-radius(4px);
 }
 .nav-tabs.nav-stacked > li > a:hover {
   border-color: #ddd;