Fixes serialization bug in github-actions OP (GithubOp)

[EthanHeilman]

When memguard was added to the GitubOP in PR #48, it introduce a bug that caused the GithupOP to throw an error when unmarshalling the response from github.

var jwt struct {
	Value *memguard.LockedBuffer
}
err = json.Unmarshal(rawBody, &jwt)
memguard.WipeBytes(rawBody)

The bug was that rawBody.value is of type string, not type byte[] so this throws json: cannot unmarshal string into Go struct field .Value of type memguard.LockedBuffer.

This PR fixes the bug and adds unittests to the GithubOP to prevent regressions.