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

[Mail body
compression](https://docs.sendgrid.com/api-reference/mail-send/mail-send#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.

base_interface.go

@@ -1,6 +1,8 @@
 package sendgrid
 
 import (
+	"bytes"
+	"compress/gzip"
 	"context"
 	"errors"
 	"net/http"
@@ -61,6 +63,15 @@ func (cl *Client) Send(email *mail.SGMailV3) (*rest.Response, error) {
 // SendWithContext sends an email through Twilio SendGrid with context.Context.
 func (cl *Client) SendWithContext(ctx context.Context, email *mail.SGMailV3) (*rest.Response, error) {
 	cl.Body = mail.GetRequestBody(email)
+	if cl.Headers["Content-Encoding"] == "gzip" {
+		var gzipped bytes.Buffer
+		gz := gzip.NewWriter(&gzipped)
+		defer gz.Close()
+		if _, err := gz.Write(cl.Body); err != nil {
+			return nil, err
+		}
+		cl.Body = gzipped.Bytes()
+	}
 	return MakeRequestWithContext(ctx, cl.Request)
 }