fix gdal_addo()

[ethanplunkett]

Fix bug that prevented using config_options argument to gdal_addo()

Reproducible example:

    library(sf)
    dir = file.path(tempdir(), "gdal_addo")
    dir.create(dir)
    tif = file.path(dir, "geomatrix.tif")
    file.copy(system.file("tif/geomatrix.tif", package = "sf"), tif, overwrite = TRUE)
    gdal_addo(tif, read_only = TRUE, config_options = c(COMPRESS_OVERVIEW="LZW"))

Previously threw error: Error: config_options should be a character vector with names, as in c(key="value")

The problem was caused by calling as.character(config_options) which stripped the names before passing it on to CPL_gdaladdo()

[edzer]

Great, thanks!