Update README and change signature for Arr::flatten

Author: cdtweb

README.md

@@ -242,4 +242,14 @@ Validator::isstring('Hello world!');
 
 // This is valid because 'String' starts with uppercased letter.
 Validator::isString('Hello world!');
- ```
+ ```
+ 
+### `\Noname\Common\Arr`
+
+A helper library for working with arrays. 
+
+#### Arr Methods
+
+##### `Arr::flatten(array $array, string $separator = '.', string $prepend = '') : array`
+
+Flatten an associative array using a custom separator.

src/Arr.php

@@ -14,17 +14,17 @@ class Arr
      * By default this method will use a dot (.) as the separator.
      *
      * @param array $array
-     * @param string $prepend
      * @param string $separator
+     * @param string $prepend
      * @return array
      */
-    public static function flatten(array $array, string $prepend = '', string $separator = '.'): array
+    public static function flatten(array $array, string $separator = '.', string $prepend = ''): array
     {
         $flatArray = [];
 
         foreach ($array as $key => $value) {
             if (is_array($value) && !empty($value)) {
-                $flatArray += self::flatten($value, $prepend . $key . $separator, $separator);
+                $flatArray += self::flatten($value, $separator, $prepend . $key . $separator);
             } else {
                 $flatArray[$prepend . $key] = $value;
             }