:= adds factor-levels silently

Start by reproducing the data.frame and data.table factor assignments shown in the issue, then inspect the implementation of := for factor columns. Compare the observed silent level addition with base R behavior and existing related work in #2403. Done means the intended behavior is established and covered for assigning a new factor level, with the issue's expected warning or error behavior made explicit.

Consider the following example data:

df <- data.frame(x = rep(c("A","B"), 2), y = 1:4)
dt <- as.data.table(df)

Now when I want to change the 3rd value of x in df to C, I will get an error with the dataframe:

> df$x[df$y == 3] <- "C"
Warning message:
In `[<-.factor`(`*tmp*`, df$y == 3, value = c(1L, 2L, NA, 2L)) :
  invalid factor level, NA generated

However when I want to this with the datable using := to update, there is no error-message (or a warning) and C is added as a factor level.

> str(dt$x)
 Factor w/ 2 levels "A","B": 1 2 1 2
> dt[y == 3, x := "C"][]
   x y
1: A 1
2: B 2
3: C 3
4: B 4
> str(dt$x)
 Factor w/ 3 levels "A","B","C": 1 2 3 2

To me this is unexpected behavior. I expected an error like when using a dataframe. When this is intended behavior, a warning would be appropriate.

Possibly related to #2403

Session info:

R version 3.6.1 (2019-07-05)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Mojave 10.14.6

Matrix products: default
BLAS:   /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib

locale:
[1] nl_NL.UTF-8/nl_NL.UTF-8/nl_NL.UTF-8/C/nl_NL.UTF-8/nl_NL.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] bench_1.0.4           microbenchmark_1.4-7  data.table_1.12.2     ggplot2_3.2.1         anytime_0.3.6         splitstackshape_1.4.8 zoo_1.8-6            
 [8] lubridate_1.7.4       purrr_0.3.2           readr_1.3.1           tibble_2.1.3          tidyr_1.0.0           dplyr_0.8.3          

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.2       pillar_1.4.2     compiler_3.6.1   tools_3.6.1      zeallot_0.1.0    lifecycle_0.1.0  gtable_0.3.0     lattice_0.20-38  pkgconfig_2.0.3  rlang_0.4.0     
[11] rstudioapi_0.10  withr_2.1.2      stringr_1.4.0    vctrs_0.2.0      hms_0.5.1        grid_3.6.1       tidyselect_0.2.5 glue_1.3.1       R6_2.4.0         magrittr_1.5    
[21] backports_1.1.4  scales_1.0.0     assertthat_0.2.1 colorspace_1.4-1 stringi_1.4.3    lazyeval_0.2.2   munsell_0.5.0    crayon_1.3.4