Reindexing grouped data frames only update class if not grouped already

[krlmlr]

to retain existing classes. Closes #3310.

Does the comment make sense?

[romainfrancois]

I'm fine with the comment.

Isn't r_match too big a 🔨 ?

btw, there is this function in tbl_cpp.h

template <typename Data>
inline Rcpp::CharacterVector classes_grouped() {
  return Rcpp::CharacterVector::create("grouped_df", "tbl_df", "tbl", "data.frame");
}

[krlmlr]

Thanks. Yeah, should be Rf_inherits(). Will update, and also use the helper.

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