Changing email object after sending mutates mail in mail.outbox

[CheesyPhoenix]

When testing emails using the locmem email backend with mail.outbox, modifying an email object after calling .send() also modifies the email object in django.core.mail.outbox. This leads to inconsistencies between test and production environments, where an email modified in production after calling .send() will not be changed since it has already been sent.

Steps to reproduce:

• Run this test in any django project:

  def test_mutate_after_send(self) -> None:
      email = EmailMessage(
          subject="correct subject",
          body="test body",
          from_email="from@example.com",
          to=["to@example.com"],
      )
      email.send()
      email.subject = "incorrect subject"
      self.assertEqual("correct subject", mail.outbox[0].subject)
  

I have already implemented a fix on a fork and will open a PR soon.