seededlda:::tfm errors if there is x in docvars

[koheiw]

> require(quanteda)
> dict <- dictionary(list('A' = "a"))
> dat <- data.frame(text = c("a b c", "A B C"), x = c(1, 2))
> corp <- corpus(dat)
> toks <- tokens(corp)
> dfmt <- dfm(toks)
> seededlda:::tfm(dfmt, dict)
Error: ndoc() only works on corpus, dfm, spacyr_parsed, tokens, tokens_xptr objects.