Hello, 

I found that when using `rbindlist` on a list of data.frames with factor columns, 
the factor column is getting concat'd as its numeric equivalent. 
'
This of course, does not happen when using a list of data.tables.  

    # sample data, using data.frame
    set.seed(1)
    sampleList.DF <- lapply(LETTERS[1:5], function(L) 
      data.frame(Val1=rnorm(3), Val2=runif(3), FactorCol=L) )
    sampleList.DF <- lapply(sampleList.DF, function(x) 
      {x$StringCol <- as.character(x$FactorCol); x})

    # sample data, using data.table
    set.seed(1)
    sampleList.DT <- lapply(LETTERS[1:5], function(L) 
      data.table(Val1=rnorm(3), Val2=runif(3), FactorCol=L) )
    sampleList.DT <- lapply(sampleList.DT, function(x) 
       x[, StringCol := as.character(FactorCol)])

lapply(sampleList.DF, data.table) -> sampleList.DT2

# Compare the column `FactorCol`: 

    rbindlist(sampleList.DT)
    rbindlist(sampleList.DT2)
    rbindlist(sampleList.DF)
    do.call(rbind, sampleList.DF)

Interestingly, I originally thought it was levels dependent: 
(I would have expected, for example, the following to allow for the levels of the third list element, but it does not).

    sampleList.DF[[1]][, "FactorCol"] <- factor(c("A", "C", "A"))
    
    # all the levels in third element are present in the first
    all(levels(sampleList.DF[[3]][, "FactorCol"])  %in%  levels(sampleList.DF[[1]][, "FactorCol"]))
    # [1] TRUE

But... 

    rbindlist(sampleList.DF)

However: 

    sampleList.DF[[1]][, "FactorCol"] <- factor(c("C", "A", "A"), levels=c("C", "A"))
    rbindlist(sampleList.DF)
    
Is the above behavior intended? 

Cheers, 
Rick