Doc updates, fix test helper

[justin808]

Test helper for detecting stale bundles did not properly handle the case
of a server-bundle.js without a hash.

This is reproducible:

1. git clone https://github.com/shakacode/react-webpack-rails-tutorial
2. bundle && yarn
3. rspec

See that the server-bundle.js is reported as missing.

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This change is 

[mapreal19]

Few minor suggestions

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks broke.

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lib/react_on_rails/prerender_error.rb, line 6 at r1 (raw file):

module ReactOnRails
  class PrerenderError < ::ReactOnRails::Error
    # TODO: Consider remove providing original `err` as already have access to `self.cause`

do we create an issue so we don't forget and maybe a newcomer could take this?

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lib/react_on_rails/version_syntax_converter.rb, line 5 at r1 (raw file):

require_relative "version"

module ReactOnRails

👍

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spec/react_on_rails/configuration_spec.rb, line 11 at r1 (raw file):

    let(:using_webpacker) { false }

    after do

I'd put the after block after the before block

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spec/react_on_rails/utils_spec.rb, line 87 at r1 (raw file):

        end

        it { expect(subject).to end_with("public/webpack/development/webpack-bundle.js") }

nice, wasn't aware of end_with

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spec/react_on_rails/test_helper/webpack_assets_status_checker_spec.rb, line 60 at r1 (raw file):

      context "when using webpacker, a missing server bundle without hash, and manifest is current" do
        let(:webpack_generated_files) { %w[manifest.json server-bundle.js] }

not for this PR but for tests in the future I'd try to stay of using let for setup.

Is a cop in the rubocop-rspec: https://github.com/rubocop-hq/rubocop-rspec/blob/master/lib/rubocop/cop/rspec/let_setup.rb#L31

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Comments from Reviewable

[coveralls]

Coverage remained the same at ?% when pulling d85d8c5 on fix-test-helper-for-server-bundle into aaef7c0 on master.

[justin808]

broke into #1102 and #1101