Provided a workaround for the JSDom issue that strips out the DOCTYPE.

JSDom will strip out your DOCTYPE, and, consequently, so does Juice. This is a [known issue](https://github.com/nodejitsu/docs/issues/49) with JSDom that doesn't look like it will be resolved anytime soon. tmpvar, the creator of JSDom, suggests a workaround for solving the problem in the above link. That workaround is what I've implemented here.