Use normal replacement for @ in slugify

src/Stringy.php

@@ -1166,6 +1166,7 @@ class Stringy implements Countable, IteratorAggregate, ArrayAccess
     {
         $stringy = $this->toAscii();
 
+        $stringy->str = str_replace('@', $replacement, $stringy);
         $quotedReplacement = preg_quote($replacement);
         $pattern = "/[^a-zA-Z\d\s-_$quotedReplacement]/u";
         $stringy->str = preg_replace($pattern, '', $stringy);

tests/StringyTest.php

@@ -1108,7 +1108,7 @@ class StringyTestCase extends PHPUnit_Framework_TestCase
             ['another-foo-bar', 'another..& foo -.-"-...bar'],
             ['foo-dbar', " Foo d'Bar "],
             ['a-string-with-dashes', 'A string-with-dashes'],
-            ['userhost', 'user@host'],
+            ['user-host', 'user@host'],
             ['using-strings-like-foo-bar', 'Using strings like fòô bàř'],
             ['numbers-1234', 'numbers 1234'],
             ['perevirka-ryadka', 'перевірка рядка'],