feat!: Gzip compress request body when content-encoding is set to gzip

[Bankq]

Fixes

Mail body compression is enabled on V3Mail. This patch added the support in rest package that when a request header has Content-Encoding: gzip set, BulidRequestObject will gzip the request body.

Checklist

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[Bankq]

To be honest I'm not entirely sure if this is appropriate.

My main use case is through https:://github.com/sendgrid/sendgrid-go . Alternatively I can make the change there instead so rest package doesn't modify the passed in body.

[Bankq]

Nvm, it turns out that every call in sendgrid-go:Client.Send is public, I can shadow it and bend it to my need.