Wrap Branch in try finally

src/Microsoft.AspNet.Http.Abstractions/Extensions/MapMiddleware.cs

@@ -57,11 +57,16 @@ namespace Microsoft.AspNet.Builder.Extensions
                 context.Request.PathBase = pathBase + _options.PathMatch;
                 context.Request.Path = remainingPath;
 
+                try
+                {
                     await _options.Branch(context);
-
+                }
+                finally
+                {
                     context.Request.PathBase = pathBase;
                     context.Request.Path = path;
                 }
+            }
             else
             {
                 await _next(context);