New Release of oauth: Version 0.5.7

oauth version 0.5.7 is Released

Setup Rubocop (#205, #208 by @pboling)
Added CODE_OF_CONDUCT.md (#217, #218 by @pboling)
Added FUNDING.yml (#217, #218 by @pboling)
Added Client Certificate Options: :ssl_client_cert and :ssl_client_key (#136, #220 by @pboling)
Handle a nested array of hashes in OAuth::Helper.normalize (#80, #221 by @pboling)

Switch from TravisCI to Github Actions (#202, #207, #176 by @pboling)
Upgrade webmock to v3.14.0 (#196 by @pboling)
Upgrade em-http-request to v1.1.7 (#173 by @pboling)
Upgrade mocha to v1.13.0 (#193 by @pboling)
HISTORY renamed to CHANGELOG.md, and follows Keep a Changelog (#214, #215 by @pboling)
CHANGELOG, LICENSE, and README now ship with packaged gem (#214, #215 by @pboling)
README.rdoc renamed to README.md (#217, #218 by @pboling)
Require plaintext signature method by default (#135 by @confiks & @pboling)

Fixed Infinite Redirect in v0.5.5, v0.5.6 (#186, #210 by @pboling)
Fixed NoMethodError on missing leading slash in path (#194, #211 by @pboling)
Fixed NoMethodError on nil request object (#165, #212 by @pboling)
Fixed Unsafe String Comparison (#156, #209 by @pboling and @drosseau)
Fixed typos in Gemspec (#204, #203, #208 by @pboling)
Copyright Notice in LICENSE - added correct years (#217, #218 by @pboling)
Fixed request proxy Class constant reference scopes - was missing :: in many places (#225, #226 by @pboling)