feat: gzip mail body when content-encoding is set to gzip

[Bankq]

Mail body compression is supported on V3 api. This patch added the support that when a client sets the content-encoding header to gzip, send compresses the mail body with gzip.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
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• I have added tests that prove my fix is effective or that my feature works
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• I have added inline documentation to the code I modified

[Bankq]

Hi @SendGridDX team, can someone comment on whether this is the desired behavior/feature of the SDK? I plan to add on tests/docs once get a vibe check

[kridai]

Hi @SendGridDX team, can someone comment on whether this is the desired behavior/feature of the SDK? I plan to add on tests/docs once get a vibe check

Hey @Bankq changes looks good to me, you can go ahead add the test and doc changes

[Bankq]

Hi @SendGridDX team, can someone comment on whether this is the desired behavior/feature of the SDK? I plan to add on tests/docs once get a vibe check

Hey @Bankq changes looks good to me, you can go ahead add the test and doc changes

Thank you @kridai !

I've added the test against it. Local docker test and a prod test against api.sendgrid.com using my own API key both works as expected.

Regarding doc, I don't find any current doc mentioning Client.Send or Client.SendWithContext. They all seems to be using API() directly by manually constructing request playload. Is this intentional? Do you want me to add the doc around the usage of Client and its methods?