Fixed SSVE support for conditionals on ViewBag

This SSVE markup:
```
@Context.ViewBag.HaveMessage;<br />
@If.Context.ViewBag.HaveMessage;
    Yay Message!
@EndIf;
```

**Produced in Nancy:**
```
True
```

**Produced in SSVE test:**
```
True
Yay Message!
```

```
[Fact]
public void Should_evaluate_context_conditional()
{
    const string input = @"@Context.ViewBag.HaveMessage;! @If.Context.ViewBag.HaveMessage;Yay message!@EndIf;";
    var context = new { ViewBag = (dynamic)new DynamicDictionary() };
    context.ViewBag.HaveMessage = true;

    ((FakeViewEngineHost)this.fakeHost).Context = context;

    var output = viewEngine.Render(input, null, this.fakeHost);

    Assert.Equal(@"True! Yay message!", output);
}
```