`survFit()` fails for single row in `newdata`

[hfrick]

Getting a survival curve via survFit() for just one observation in newdata fails. I think it's just a matter of not dropping from a 1-column matrix to a vector in the expression before that. I'll send a PR :raised_hands:

library(mboost)
#> Loading required package: parallel
#> Loading required package: stabs
library(survival)

mod <- blackboost(Surv(time, status) ~ age + ph.ecog, 
                  data = lung, 
                  family = CoxPH())
survFit(mod, lung[1,])
#> Error in `colnames<-`(`*tmp*`, value = rownames(newdata)): attempt to set 'colnames' on an object with less than two dimensions

^{Created on 2022-05-11 by the reprex package (v2.0.1)}