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Merge pull request wechatpay-apiv3#46 from wechatpay-apiv3/hotfix/fix…

…_get_serial_number

修复获取证书序列号时会丢弃其头部0的问题

Merge pull request wechatpay-apiv3#44 from wechatpay-apiv3/feature/pa…

…rtnerpayments

增加服务商支付接口 `partnerpayments`

Merge pull request wechatpay-apiv3#36 from wechatpay-apiv3/test/fix_s…

…ervices_certificates

test: 更新 api_certificates_test.go，使其不再依赖真实环境。