Merge branch 'kn/osxkeychain-idempotent-store-fix' into next

An earlier check added to osx keychain credential helper to avoid
storing the credential itself supplied was overeager and rejected
credential material supplied by other helper backends that it would
have wanted to store, which has been corrected.

* kn/osxkeychain-idempotent-store-fix:
  osxkeychain: avoid incorrectly skipping store operation