merged branch kriswallsmith/kernel/unnecessary-regex (PR #3104)

Commits
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7f7f82a [HttpKernel] removed unnecessary regex

Discussion
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[HttpKernel] removed unnecessary regex

The pattern was also flawed because of the unescaped `.`

```
Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
```

src/Symfony/Component/HttpKernel/HttpCache/Esi.php

@@ -64,7 +64,7 @@ class Esi
             return false;
         }
 
-        return (Boolean) preg_match('#ESI/1.0#', $value);
+        return false !== strpos($value, 'ESI/1.0');
     }
 
     /**