nl2br - fix inline string

From Twig/test/Twig/Tests/Fixtures/filters/nl2br.test:
{{ "I like Twig.\nYou will like it too.\n\nEverybody like it!"|nl2br }}

Should have become:
You will like it too.<br />
<br />
Everybody like it!

But was:
I like Twig.\nYou will like it too.\n\nEverybody like it!

This is because newlines were escaped in string literals like that, this
patch handle those as well