Fix randomly failing test spec/features/users_spec.rb

[nalabjp]

fix #200

明確な原因まではわからなかったが、Scrivito::Workspace.currentの状態が変わることがあるようで、
それによりObjが見つからないケースが有る模様。（WorkspaceとObjの関係性の理解が足りていない😇）

letを使うと--seed 49001が手元で通るようになるので、spec実行時はScrivtio::Workspace.currentが意図したものが取れているような感じ。

~/ghq/github.com/nalabjp/coderdojo.jp fix-user-spec
$ git rev-parse HEAD
17b538a0448b668a8fcde51c0209deecb026a161
~/ghq/github.com/nalabjp/coderdojo.jp fix-user-spec
$ bundle exec rspec spec/ --seed 49001

Randomized with seed 49001
................................................

Top 10 slowest examples (11.04 seconds, 92.9% of total time):
  Sotechsha GET /sotechsha/num Quizzes should be permalink
    2.32 seconds ./spec/features/sotechsha_spec.rb:7
  Sotechshas Quizzes should be permalink Quizzes should be permalink
    2.15 seconds ./spec/requests/sotechshas_spec.rb:6
  Users log in with invalid information should not text "道場情報まとめ"
    2.07 seconds ./spec/features/users_spec.rb:24
  Users log in with invalid information should text "ログイン"
    1.33 seconds ./spec/features/users_spec.rb:23
  Users log in with valid information should text "道場情報まとめ"
    1.3 seconds ./spec/features/users_spec.rb:33
  NewsPages GET /news/2016/12/12/new-backend Blog post should be rendered
    0.43522 seconds ./spec/requests/news_pages_spec.rb:5
  SessionsController GET #create param unmatch
    0.38812 seconds ./spec/controllers/sessions_controller_spec.rb:10
  Sotechshas GET /sotechsha/gazou Gazoulink should be permalink
    0.35926 seconds ./spec/requests/sotechshas_spec.rb:15
  News GET /news/2016/12/12/new-backend Title should be formatted
    0.3502 seconds ./spec/features/news_spec.rb:6
  Sotechsha GET /sotechsha/1 Datetime should be formatted
    0.3476 seconds ./spec/features/sotechsha_spec.rb:26

Top 10 slowest example groups:
  Users
    1.56 seconds average (4.69 seconds / 3 examples) ./spec/features/users_spec.rb:5
  Sotechshas
    1.25 seconds average (2.51 seconds / 2 examples) ./spec/requests/sotechshas_spec.rb:3
  Sotechsha
    0.99368 seconds average (2.98 seconds / 3 examples) ./spec/features/sotechsha_spec.rb:4
  NewsPages
    0.43557 seconds average (0.43557 seconds / 1 example) ./spec/requests/news_pages_spec.rb:3
  News
    0.35053 seconds average (0.35053 seconds / 1 example) ./spec/features/news_spec.rb:4
  Statistics::Aggregation
    0.13068 seconds average (0.13068 seconds / 1 example) ./spec/lib/statistics/aggregation_spec.rb:4
  SessionsController
    0.10376 seconds average (0.41503 seconds / 4 examples) ./spec/controllers/sessions_controller_spec.rb:3
  Top
    0.04013 seconds average (0.04013 seconds / 1 example) ./spec/features/top_spec.rb:3
  SotechshaOverviewPageController
    0.03893 seconds average (0.03893 seconds / 1 example) ./spec/controllers/sotechsha_overview_page_controller_spec.rb:3
  DocsController
    0.03785 seconds average (0.11356 seconds / 3 examples) ./spec/controllers/docs_controller_spec.rb:3

Finished in 11.89 seconds (files took 6.45 seconds to load)
48 examples, 0 failures

Randomized with seed 49001

[yasulab]

PRありがとうございます！マージしますね :)