Restore memoize mistakenly removed in 7f412a16d72bad13b
Revert "Restore memoize mistakenly removed in 7f412a16d72bad13b"

This reverts commit 29125cb1cc733b0aaf149bad538951ca7fa7c3b6.
Revert "Cleanup Octokit::Client#request"

This reverts commit 7f412a16d72bad13b1fcf386fc9ba2e5c5b73ea4.
more test coverage for #exchange_code_for_token.
note default values for #exchange_code_for_token.
don't use nil values when testing a method's default arguments (see #330).
stub body parameters for .exchange_code_for_token (see #330).
Cleanup improved #exchange_code_for_token specs
Merge branch 'eturk-exchange_code_for_token-specs'