Support running in JVM bootstrap class loader

Hi, I'd like to use undertow in a java agent that runs in the bootstrap class loader.  This causes NullPointerExceptions in a couple of places because getClassLoader() returns null for classes in the bootstrap class loader, e.g.

```
HttpResponseParser.class.getClassLoader().loadClass("...")
```

This pull request uses Class.forName() instead, which works equally well in the bootstrap class loader, e.g.

```
Class.forName("...", false, HttpResponseParser.class.getClassLoader())
```