fix: added nil to empty values

[ABeltramo]

I changed the zipmap default method so that it will always create a map with all the passed keys.
Another solution can be:

(defn pad [n coll val]
        (take n (concat coll (repeat val))))

(defn mappify-row
  "Translates a single row of values into a map of `colname -> val`, given colnames in `header`."
  [header row]
  (zipmap header (pad (count header) row nil))) ; here we can replace nil with any default value passed by the user

Fixes #65

[metasoarous]

Thanks for the PR!

See comments in #65; This is great, but would you please make this functionality opt-in only?

Thanks again

[ABeltramo]

There you go, added :preserve-header to the mappify options.