Fix a bug in `margins()` with `glmerMod` objects (#129)

[vincentarelbundock]

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In combination with my concurrent PR at leeper/margins this fixes Issue #129 of margins.

Here, I am simply improving find_data.merMod. I have written no new code for this. I just copied over the find_data.default method, and modified the model[['class']] by model@class, because glmerMod is a S4.

[leeper]

thanks!