Always escape the result of link_to_unless method

The return string of link_to_unless method should be always html-escaped to keep consistency.

```ruby
link_to_unless(false, '<b>Showing</b>'.html_safe, 'github.com')
# => "<a href=\"github.com\"><b>Showing</b></a>"

link_to_unless(true, '<b>Showing</b>'.html_safe, 'github.com')
# => "<b>Showing</b>"

link_to_unless(false, '<b>Showing</b>', 'github.com')
# => "<a href=\"github.com\">&lt;b&gt;Showing&lt;/b&gt;</a>"

link_to_unless(true, '<b>Showing</b>', 'github.com')
# => "<b>Showing</b>" should be "&lt;b&gt;Showing&lt;/b&gt;"
```

Could you consider to merge this request?