Uses of FaradayMiddleware#on_complete should not be private

[tarebyte]

Fixes #1315

In lostisland/faraday#1194 on_complete was introduced to Faraday::Middleware and was extracted out of Faraday::Response. Because our implementations of on_complete were private they no longer responded to FaradayMiddleware#call.

This updates all uses of Octokit::Response::Middleware so that on_complete isn't a private method.

[tarebyte]

For posterity

~/src/octokit/octokit.rb tarebyte/faraday-fixes                                                                                                                                                                                                                                       12s 15:15:44
❯ bundle info faraday
The dependency jruby-openssl (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for java. To add those platforms to the bundle, run `bundle lock --add-platform java`.
The dependency json (>= 2.3.0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for java. To add those platforms to the bundle, run `bundle lock --add-platform java`.
  * faraday (1.2.0)
        Summary: HTTP/REST API client library.
        Homepage: https://lostisland.github.io/faraday
        Path: /Users/tarebyte/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/faraday-1.2.0
~/src/octokit/octokit.rb tarebyte/faraday-fixes                                                                                                                                                                                                                                           15:15:49
❯ bundle exec rake
/Users/tarebyte/.rbenv/versions/2.6.5/bin/ruby -I/Users/tarebyte/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rspec-core-3.10.1/lib:/Users/tarebyte/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rspec-support-3.10.1/lib /Users/tarebyte/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rspec-core-3.10.1/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb

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Finished in 27.37 seconds (files took 1.02 seconds to load)
818 examples, 0 failures

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Coverage report generated for RSpec to /Users/tarebyte/src/octokit/octokit.rb/coverage. 7231 / 7282 LOC (99.3%) covered.
~/src/octokit/octokit.rb tarebyte/faraday-fixes