fixes #4254: make .span12 expand full width of a parent when responsive

docs/assets/css/bootstrap-responsive.css

@@ -814,6 +814,13 @@
     width: auto;
     margin-left: 0;
   }
+  .span12,
+  .row-fluid .span12 {
+    width: 100%;
+    -webkit-box-sizing: border-box;
+       -moz-box-sizing: border-box;
+            box-sizing: border-box;
+  }
   .input-large,
   .input-xlarge,
   .input-xxlarge,

less/responsive-767px-max.less

@@ -62,6 +62,11 @@
     width: auto;
     margin-left: 0;
   }
+  .span12,
+  .row-fluid .span12 {
+    width: 100%;
+    .box-sizing(border-box);
+  }
 
   // FORM FIELDS
   // -----------