Check the GitHub API:

The merge_queue.merge_method only supports the upper case: MERGE, SQUASH, REBASE

However, in the code the enum is in lower case, which easily mislead users to pass in the enum in lowercase. I have tested from my side, lower case will cause GitHub server return an error directly:
https://github.com/google/go-github/blob/v71.0.0/github/rules.go#L106

Note: I understand for other type of rulesets, it requires lowercase again which is an issue of GitHub API. They should have made case consistent 🤦 :

go-github should still make things clearer: add uppercase enum and comments about when to use lowercase and not.