default parameters aren't saved in save_trial_parameters

[jadeddelta]

while browsing the implementation of css_classes, i noticed that this (along with other the parameters that are present in all plugins) aren't able to be saved via the save_trial_parameters field. examples/css-classes-parameter.html recreates this, as running the trial gives a warning that css_classes does not exist for each trial, and it doesn't appear in the final data.