[web] Avoid using an empty dict as default parameter of 'CubicWebRequestBase'.
Also update the related docstring of the __init__ method.
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0