bug: in tcc mode, json.Unmarshal requestContext.Headers failed

[dk-lockdown]

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#12

In tcc mode, When initiating a commit or rollback request, json.Unmarshal requestContext.Headers failed:

2022-07-11T07:25:20.106Z        ERROR   dt/distributed_transaction_manger.go:419        error doHttpRequest for tccBranchCommit: error json.Unmarshal requestContext.Headers: invalid character 'P' looking for beginning of value
github.com/cectc/dbpack/pkg/dt.(*DistributedTransactionManager).processNextBranchSession
        /app/pkg/dt/distributed_transaction_manger.go:419
github.com/cectc/dbpack/pkg/dt.(*DistributedTransactionManager).processBranchSessionQueue
        /app/pkg/dt/distributed_transaction_manger.go:390

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