get-column-id returns keyword version of column-key if convenient

If given column key is a string which is not used in a dataset, and a keyword version of this column-key is used in the dataset - return keyword version.

Example:
```Clojure
user>(def ds (dataset [:a :b :c] [[1 2 3] [4 5 6]]))
user> (sel ds :cols "a")
(1 4)
user> (sel ds :cols ["a" "b"])
| :a | :b |
|----+----|
|  1 |  2 |
|  4 |  5 |
```