Fix TSR for empty path

Resulted in a panic before. Fixes #45

router.go

diff --git a/router.go b/router.go
index a6be43c..7716dc7 100644
--- a/router.go
+++ b/router.go
@@ -289,7 +289,7 @@
 			}
 
 			if tsr && r.RedirectTrailingSlash {
-				if path[len(path)-1] == '/' {
+				if len(path) > 1 && path[len(path)-1] == '/' {
 					req.URL.Path = path[:len(path)-1]
 				} else {
 					req.URL.Path = path + "/"

router_test.go

diff --git a/router_test.go b/router_test.go
index 08dab29..19aeb39 100644
--- a/router_test.go
+++ b/router_test.go
@@ -171,6 +171,7 @@
 	router := New()
 	router.GET("/path", handlerFunc)
 	router.GET("/dir/", handlerFunc)
+	router.GET("/", handlerFunc)
 
 	testRoutes := []struct {
 		route  string
@@ -179,6 +180,7 @@
 	}{
 		{"/path/", 301, "map[Location:[/path]]"},   // TSR -/
 		{"/dir", 301, "map[Location:[/dir/]]"},     // TSR +/
+		{"/dir", 301, "map[Location:[/dir/]]"},     // TSR +/
 		{"/PATH", 301, "map[Location:[/path]]"},    // Fixed Case
 		{"/DIR/", 301, "map[Location:[/dir/]]"},    // Fixed Case
 		{"/PATH/", 301, "map[Location:[/path]]"},   // Fixed Case -/