Fix #522, [MusicStore]: Html.BeginForm() while generating the action url

should consider the application base path
- `request.BasePath` was indeed ignored
- also simplify `formAction` calculation using the higher-level
  `PathString.Add()` overloads

src/Microsoft.AspNet.Mvc.Core/Rendering/Html/HtmlHelper.cs

@@ -769,19 +769,7 @@ namespace Microsoft.AspNet.Mvc.Rendering
                 // parameters. Also reachable in the even-more-unusual case that user called another BeginForm()
                 // overload with default argument values.
                 var request = ViewContext.HttpContext.Request;
-                if (request.Path.HasValue)
-                {
-                    formAction = request.Path.Value;
-                }
-                else
-                {
-                    formAction = string.Empty;
-                }
-
-                if (request.QueryString.HasValue)
-                {
-                    formAction += request.QueryString.Value;
-                }
+                formAction = request.PathBase.Add(request.Path).Add(request.QueryString);
             }
             else
             {