Make Headers::get case insensitive

Because if a header is set like this: `Content-type: application/json`

```
Headers::get('Content-Type');  // null
Headers::get('Content-type');  // 'application/json'
```

Causes problems in the `Request::get` method because it looks exactly for `Content-Type`.

Author: induwara16

src/Headers.php

@@ -64,7 +64,7 @@ public static function all(bool $safeOutput = false): array
      */
     public static function get($params, bool $safeOutput = false)
     {
-        if (is_string($params)) return self::all($safeOutput)[$params] ?? null;
+        if (is_string($params)) return array_change_key_case(self::all($safeOutput), CASE_LOWER)[strtolower($params)] ?? null;
 
         $data = [];
         foreach ($params as $param) {