Change default value of limit_posts from nil to 0 (see #1004)

Before this commit meaning of limit_posts:
  nil: no limit. generate all posts
  0: raise error
  n ( > 0): generate n posts only
  n ( < 0): raise error
  else: raise error

After this commit:
  nil: same as 0
  0: no limit. generate all posts
  n ( > 0): generate n posts only
  n ( < 0): raise error
  else: almost same as 0 (depend on `to_i` result)