Fix the issue that the first header is still right aligned

[shrektan]

Fix the issue that the first header is still right aligned when the data has no row name and the target is set to "_all". It's a patch for #512 .

some examples

DT::datatable(
  data.frame(a = 1, b = 2, c = 3, d = 4),
  options = list(
    columnDefs = list(
      list(className = "dt-center", targets = "_all")
    )
  )
)

DT::datatable(
  data.frame(a = 1, b = 2, c = 3, d = 4),
  options = list(
    columnDefs = list(
      list(className = "dt-center", targets = c(-1, 2))
    )
  )
)


DT::datatable(
  sapply(iris[,1:3], as.integer),
  options = list(
    columnDefs = list(
      list(className = "dt-center", targets = "_all")
    )
  )
)


DT::datatable(
  iris[,1:3], rownames = FALSE,
  options = list(
    columnDefs = list(
      list(className = "dt-center", targets = "_all")
    )
  )
)

[yihui]

Thanks!