rbind.dfm() should not drop docvars

[koheiw]

cbind.dfm() drops original docvars:

quanteda/R/dfm-classes.R

Line 267 in 2f26a1b

docvars = make_docvars(nrow(x) + nrow(y), c(docnames(x), docnames(y)), unique = FALSE),

but +.tokens() keeps them

quanteda/R/tokens-methods.R

Lines 199 to 200 in d206654

	docvars <- rbind_fill(get_docvars(t1, user = TRUE, system = TRUE),
	get_docvars(t2, user = TRUE, system = TRUE))