fixed bug that 'with' spec is invalid.

I fixed issue #314 bug 

So, following test code is success about 'with' spec.

```
@Test
public void withHelperSpec() throws IOException {
    String context = "{ obj: { context: { one: 1, two: 2 } } }";

    shouldCompileTo("{{#with obj}}{{context.one}}{{/with}}",     context, "1");
    shouldCompileTo("{{#with obj}}{{obj.context.one}}{{/with}}", context, "");
}
```