sql server: fix writing to tmp tables with FreeTDS

[detule]

Hi:

This fixes an issue seen when writing to temp tables in SQL Server when using FreeTDS:

val <- paste(rep(paste(LETTERS, collapse = ""), 100), collapse = "")
> nchar(val)
[1] 2600
dbWriteTable(conn, "#test_freetds_tmp_write", data.frame(col = val))
res <- DBI::dbReadTable(conn, "#test_freetds_tmp_write")
> nchar(res[[1]])
[1] 255
---------------------------
> conn@info$drivername
[1] "libtdsodbc.so"
> conn@info$driver.version
[1] "01.01.9999"

It introduces an additional query or two when writing ( to temp tables ), but I think the cost is sufficiently low that I am not too concerned about it yet. Besides, without this patch, odbcConnectionColumns is genuinly broken for temp tables / doesn't work for any driver; it's just that the OEM driver can manage the write despite us not being able to discern the parameter types.

[simonpcouch]

I'm with it!