openapi2conv: preserve x-fields when converting from v2 to v3#1092

fenollp merged 2 commits intogetkin:mastergetkin/kin-openapi:masterfrom

saltbo:fix/preserve-extensions-in-v2-to-v3-conversionsaltbo/kin-openapi:fix/preserve-extensions-in-v2-to-v3-conversionCopy head branch name to clipboard