Add Date header to sent e-mails

[pcouy]

Should resolve #1140

I have tested this using Dockerfile-build. I replaced image: binwiederhier/ntfy with the following in my docker-compose.yml :

build:
  context: ./ntfy
  dockerfile: Dockerfile-build

E-mails are not rejected by my mail server anymore.

[pcouy]

Hi, thanks for the approval. Any update on getting this merged ?

[binwiederhier]

Thanks!