Always Escape the Return String of link_to_unless

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