请求转发过程中，有时候需要获取当前请求头，以附加更多的请求头或者移除某些请求头


`c#
 saga.SetBranchHeaders(new System.Collections.Generic.Dictionary<string, string> { { "x-tenant-id","1" }, { "Auhthorization","1" } });
 saga.SetBranchHeaders(new System.Collections.Generic.Dictionary<string, string> { { { "Auhthorization","1" } });//直接覆盖了x-tenant-id 
`
需要更灵活地控制BranchHeader