fix test helper

[justin808]

Review in #1093.

This change is 

[coveralls]

Coverage remained the same at ?% when pulling ca2c1b3 on fix-test-helper into 1605b40 on master.

[justin808]

@mapreal19 maybe skim this PR...just tiny changes beyond what you reviewed. Serious yak shave on the need to add this to the test!

after do
      ReactOnRails::Utils.instance_variable_set(:@server_bundle_path, nil)
    end

[mapreal19]

1 minor comment

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Reviewed 10 of 10 files at r1.
Review status: all files reviewed, 1 unresolved discussion (waiting on @justin808)

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lib/react_on_rails/webpacker_utils.rb, line 11 at r1 (raw file):

      # [2] (pry) ReactOnRails::WebpackerUtils: 0> Webpacker.manifest.lookup("app-bundle.js")
      # "/webpack/development/app-bundle-c1d2b6ab73dffa7d9c0e.js"
      # Next line will throw if the file or manifest does not exist

This info belongs to the lookup! method spec. Otherwise we would need to comment every time this method is being used.

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Comments from Reviewable