menu_tags context's stack bug

Because context.update add a new level in the Context's stack and because there is never a context.pop called in menu_tags, the context AFTER a menu_tags call is not the same than BEFORE the call and there is some funny.

## How to reproduce

onecolum.html :

```
<h1>Dolly vs Magic Pony Fight</h1>
{% block main %}
<div id="maincolumn">some content</div>
{% endblock %}
```

twocolumns.html :

```
{% extends "one_olumn.html" %}
{% load menu_tags %}
{% block main %}
    {% with template='Magic Pony is the best !' %}
        <h2>{{ template }}</h2>
        {% block sidebar %}
            <aside>
                But Dolly could be a problem !
                {% show_sub_menu 1 %}
                <h2>{{ template }}</h2>
            </aside>
        {% endblock %}
        {{ block.super }}
    {% endwith %}
{% endblock %}
```

### Expected Result

When twocolumns.html is rendered, we should have :

```
<h1>Dolly vs Magic Pony Fight</h1>
<h2>Magic Pony is the best !</h2>
<aside>
    But Dolly could be a problem !
    <h2>Magic Pony is the best !</h2>
</aside>
<div id="maincolumn">some content</div>
```

### Current Result

But when twocolumns.html is rendered, we currently have :

```
<h1>Dolly vs Magic Pony Fight</h1>
<h2>Magic Pony is the best !</h2>
<aside>
    But Dolly could be a problem !
    <h2>menu/sub_menu.html</h2>
</aside>
<aside>
    But Dolly could be a problem !
    <h2>menu/sub_menu.html</h2>
</aside>
```