Resolves ClassCastException for grid layouts

## The Problem

When running `ITextRenderer.layout()` on a grid formatted html file, you would get:

```
java.lang.ClassCastException: org.xhtmlrenderer.render.BlockBox cannot be cast to org.xhtmlrenderer.newtable.TableBox
```

A great example would be to tagsoup http://twitter.github.com/bootstrap/

## The Solution

An extra check to block the exception from bubbling to the surface. This simple change resolves:

- http://code.google.com/p/flying-saucer/issues/detail?id=144

Thank you for a such a great library!