Fix distinctOnOrderBy with nested expression

[NikitaRazmakhnin]

Should solve #277.
Code assembling DISTINCT ON clause was stripping important part of important nested expressions. It worked well without nested expressions but with them it just cuts by spaces parts with arguments. So for example with COALESCE(val, ?) ASC it splits assembled expression into ["COALESCE(val,", "?)", "ASC"] which causes bad sql for some cases.

I am not sure it is a good solution because I actually don't know is it possible to have other expressions here or not? Please, let me know if you see any other possible issues with that fix. :)

A related test-case is added.

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[parsonsmatt]

fantastic, thanks!