You ask how to avoid escaping, but it sounds like what you really want is for browsers to display "&" instead of "&".
If you use your browser's "View Source" feature on the page you said looked right, you will see that it begins with:
<!DOCTYPE html>
<html itemscope itemtype="http://schema.org/QAPage">
<head>
<title>symfony2 - Symfony 2, Twig: how not to escape field value (used with backbonejs & symfony 2) - Stack Overflow</title>
Observe that ampersand is entity-encoded as & just the way it "should" be.
Observe that the document begins with a DOCTYPE declaration. A document that begins with something else will probably trigger the browser's quirks mode for compatibility.
Use your browser's "View Source" function on your Rails application. Does the document begin with a DOCTYPE declaration? (<DOCTYPE html> is good.) Does the sequence & appear on its own? (If you see &amp; or similar, you're encoding an already-encoded string.)
