How to add a custom request header

[gooodwu]

I wanted to implement the call transfer function via gosip, via the REFER method, but could not find a way to add a custom request header，Hoping for help，thanks!

[ghettovoice]

Hello @gooodwu ,
sip.REFER is already here https://github.com/ghettovoice/gosip/blob/master/sip/message.go#L43

[github-actions]

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