Make {{ . }} notation work with sets

[dluksza]

Preciously when {{ . }} was used together with set it was producing
single output eg:

(render-string "{{# foo }} {{ . }} {{/ foo }}" {:foo #{1 2 3}})
" #{1 3 2} "

Now previous will iterate over set processing one element at a time.
This means that example mentioned above will produce:

" 1 3 2 "

Note, that the order of values is different and cannot be guaranteed.

Fixes #30