[web] Fix handling of some http headers
The generator takes as argument a list of raw string values, and is
supposed to return the parsed version. Calling str on that list makes
no sense.
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py