Fix Single Page Application example in README.md file (#678)

Fixes #588

**Summary of Changes**

1. Add test case to validate proposed fix (both negative and positive
test case).
2. Update `README.md` file.

PS: If you want to verify how I'm able to reproduce the issue and tried
my fix. Here is a link of my local PR where I have configured `Github
Action` to execute same test cases on all the platforms i.e. `ubuntu`,
`macos` and `windows`.

PR link: amustaque97#1

> PS: Make sure your PR includes/updates tests! If you need help with
this part, just ask!

---------

Co-authored-by: Corey Daley <[email protected]>

README.md

@@ -247,20 +247,11 @@ type spaHandler struct {
 // file located at the index path on the SPA handler will be served. This
 // is suitable behavior for serving an SPA (single page application).
 func (h spaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-    // get the absolute path to prevent directory traversal
-	path, err := filepath.Abs(r.URL.Path)
-	if err != nil {
-        // if we failed to get the absolute path respond with a 400 bad request
-        // and stop
-		http.Error(w, err.Error(), http.StatusBadRequest)
-		return
-	}
-
-    // prepend the path with the path to the static directory
-	path = filepath.Join(h.staticPath, path)
+    // Join internally call path.Clean to prevent directory traversal
+    path := filepath.Join(h.staticPath, path)
 
     // check whether a file exists at the given path
-	_, err = os.Stat(path)
+	_, err := os.Stat(path)
 	if os.IsNotExist(err) {
 		// file does not exist, serve index.html
 		http.ServeFile(w, r, filepath.Join(h.staticPath, h.indexPath))