create index and statistics must have quotes in columns

[jrubio75]

I add quotes on 4 lines--> add column_names because reserved words crashes like "PLAN" or whitespaces.
mppscripter.psm1
lines 418 and 421
[void]$index_cols.Add(", [$($.column_name)] $(if($.is_descending_key) {"DESC"} else {"ASC"})");
lines 453 and 456
[void]$stats_cols.Add(", [$($_.column_name)]");