desc: fix type of `limit` and `zip` on streams (#678)
tests:

  - cd: r.db('test').table_create('678')
    def: tbl = r.table('678')

  - rb: tbl.map{|x| x}.limit(1).typeof
    ot: ("STREAM")

  - rb: r([1]).map{|x| x}.limit(1).typeof
    ot: ("ARRAY")

  - cd: r.db('test').table_drop('678')
