WebResponse.getContentAsStreamWithBomIfApplicable() overriding is missing in WebResponseWrapper #471

This causes css cache loading error. In CssStyleSheet, if the response is from cache, then the response is a WebResponseWrapper(extended from WebResponseWrapper) instance. Since the method getContentAsStreamWithBomIfApplicable() is not overriden in WebResponseWrapper, it actually calls the WebResponse.getContentAsStreamWithBomIfApplicable() which always returns null because it's only an empty delegator.