desc: concatmap that doesn't return stream crashes server (#568)
tests:

  - cd: r.db('test').table_create('568')
    def: tbl = r.db('test').table('568')

  - cd: tbl.insert({'name':'Jim Brown'})

  - js: tbl.concatMap(function(rec){return rec("name")})
    py: tbl.concat_map(lambda rec:rec["name"])
    rb: tbl.concat_map {|rec| rec[:name]}
    ot: err("RqlRuntimeError", "Cannot convert STRING to SEQUENCE", [])

  - cd: r.db('test').table_drop('568')
