bug in `textmodel_NB()` when `prior = "docfreq"`

[kbenoit]

In the ?textmodel_NB example:

> nb.p261 <- textmodel_NB(trainingset, trainingclass, prior = "docfreq")
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 Error in t(PwGc) %*% Pc : 
  not-yet-implemented method for <dgeMatrix> %*% <table> 

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