merged branch jfcixmedia/2.1 (PR #5838)

This PR was squashed before being merged into the master branch (closes #5838).

Commits
-------

201f3e6 [Form] Fixed cannot unset string offsets in CsrfValidationListener

Discussion
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[Form] Fixed cannot unset string offsets in CsrfValidationListener

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: -

A php fatal error is happening when someone rewrite the entire form data for an object with a single input.
```
Fatal error: Cannot unset string offsets in vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Csrf/EventListener/CsrfValidationListener.php on line 72
```

Example:

```html
<form action="/app_dev.php/post/create" method="post" >
    <div id="posttype">
        <div>
            <label for="posttype_name" class="required">Name</label>
            <input type="text" id="posttype_name" name="posttype[name]" required="required" maxlength="255" />
        </div>
        <div>
            <label for="posttype_text" class="required">Text</label>
            <textarea id="posttype_text" name="posttype[text]" required="required"></textarea>
        </div>
        <input type="hidden" id="posttype__token" name="posttype[_token]" value="83a1617c694fbdea43c2527f1a55c7419ce82a42" /></div>
        <p>
            <button type="submit">Create</button>
        </p>
</form>
```

If someone alters the html to add a simple input at the bottom of the form like this one:
```html
<input type="text" id="posttype" name="posttype" value="test123" />
```

The result will be a php fatal error.

---------------------------------------------------------------------------

by bschussek at 2012-10-26T09:49:05Z

Thank you for the pull request! Could you please reference the pull request in the test?

```php
// https://github.com/symfony/symfony/pull/5838
public function testStringFormData()
{
    ...
```

---------------------------------------------------------------------------

by jfcixmedia at 2012-10-26T10:21:29Z

@bschussek  Added, thanks.