Unable to create filters with "+" in property name

[ml-evs]

Reported by @VicTrqt for the Materials Project (cc @tsmathis) -- you cannot construct a valid query on a field like _mp_stability.gga_gga+u_r2scan < 0.05, even when quoting or percent encoding the "+". Probably this field name should be forbidden at the optimade-python-tools level, as I don't think it is a valid OPTIMADE field name...