JFilterInput PATH filter does not support a leading slash

Test code:
```php
$filter = JFilterInput::getInstance();
var_dump($filter->clean('/path/to/joomla-cms/index.php', 'path'));
```

Expected Result: '/path/to/joomla-cms/index.php'
Actual Result: ''

Apply patch and re-test

Travis-CI job which should show unit test failure - https://travis-ci.org/mbabker/joomla-cms/builds/43885074
Travis-CI job which should pass with patch (in addition to the job for this PR :smile: ) - https://travis-ci.org/mbabker/joomla-cms/builds/43885122