[table view] allow to set None in 'headers', meaning the label should be fetched from the result set as done by default (closes #1931533)
the point of this is to avoid having to define the whole headers list when only
some of them have to be defined.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared