uniting objects with "base" resolution and strand "*" breaks when requesting destranding

[alexg9010]

library(methylKit)
data(methylKit)

mdb <- makeMethylDB(methylRawList.obj)     

# works for "base" -resolution
unite(mdb,destrand=TRUE)
mdb # is fine

# breaks for "region" -resolution
mdbt <- tileMethylCounts(mdb) 
mdbt <- methylRawListDB(lapply(mdbt, function(x) {x@resolution <- "base"; x}),treatment = getTreatment(mdb))
# works
unite(mdbt)    

unite(mdbt,destrand=TRUE)    
> destranding...
> Error in getTabixByChunk(tbxFile, chunk.size = nrow, return.type) : 
>  the tabix file seems to be empty. stopping here.

[alexg9010]

related to #282