nth slicing function doesn't work as intended

[den-kozlov]

Documentation says that nth should return n-th element from iterator, which is not true for some cases.
Example:

tarantool> require "fun"()

tarantool> nth(1, drop_while(function(t) return t ~= "5" end, "123456789")) -- expecting '5'
---
- '1'
...

tarantool> nth(1, drop_while(function(t) return t ~= 5 end, {1,2,3,4,5,6,7,8,9})) --expecting 5
---
- 1
...

tarantool> nth(1, drop_while(function(t) return t ~= 5 end, range(9))) -- now it's correct
---
- 5
...

[d1rebear]

Pull request (didn't update tests - sorry): #56.

[RunsFor]

Also smashed into this:

tarantool> require('fun').iter({1,2,3,false,5,6,7}):drop_while(function(i) return i end):take(1):totable()[1]
---
- false
...

tarantool> require('fun').iter({1,2,3,false,5,6,7}):drop_while(function(i) return i end):nth(1)
---
- 1
...