plot.OutputsModel: remove case sensitivity to the `which` argument

Is it possible to have the which argument case insensitive?:

> plot(OMinf2nat$CHALO_21, Qinf[I_Run, "CHALO_21"], which = "regime")
Error in plot.OutputsModel(OMinf2nat$CHALO_21, Qinf[I_Run, "CHALO_21"],  : 
  incorrect element found in argument 'which': "regime"
it can only contain "all", "synth", "ts", "perf", "Precip", "PotEvap", "ActuEvap", "Temp", "SnowPack", "Flows", "Error", "Regime", "CumFreq", "CorQQ"
> plot(OMinf2nat$CHALO_21, Qinf[I_Run, "CHALO_21"], which = "Regime")