Client is not safe for concurrent use

[dwrz]

Using github.com/sendgrid/sendgrid-go v3.14.0, it appears that Client is not safe for concurrent use.

I was running a service with the race detector enabled, and it detected a data race at:

github.com/sendgrid/sendgrid-go.(*Client).SendWithContext() 
/vendor/github.com/sendgrid/sendgrid-go/base_interface.go:65 +0x8c

The problem is (at least) this line:

	cl.Body = mail.GetRequestBody(email)

If the client is called concurrently, there is a data race in writing the email parameter to the Body field.

Ideally, the assignment should either be protected by a mutex, or even better, the parameter should not be assigned to any field.

Otherwise, it would be great if you could document that the client is not safe for concurrent use.