Fix survFit() for single row in newdata

[hfrick]

closes #117

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

mod <- blackboost(Surv(time, status) ~ age + ph.ecog, 
                  data = lung, 
                  family = CoxPH())
sF_1 <- survFit(mod, lung[1,])

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

[hbaniecki]

I am looking forward to seeing this merged 👍🏻