topfeatures(x, group = something) fails with weighted dfms

[kbenoit]

Discovered in https://stackoverflow.com/questions/59148580/whats-the-correct-way-to-extract-tf-idf-topfeatures-by-document/59152416?noredirect=1#comment115615693_59152416:

library("quanteda")
## Package version: 2.1.2

dfmat <- dfm(c("a b b b c c d", "a a b d d"))

topfeatures(dfmat, n = 1, groups = c("group1", "group2"))
## $group1
## b 
## 3 
## 
## $group2
## a 
## 2

topfeatures(dfm_tfidf(dfmat), n = 1, groups = c("group1", "group2"))
## Error: will not group a weighted dfm; use force = TRUE to override

Solution is to use force = TRUE by default.