only fetch RFC822 with `#message`

Hey there,

Thanks for your code, it really helped me out.

I'd really like to make it easy to integrate your PR at https://github.com/nu7hatch/gmail/pull/66, so I'm submitting this patch for your consideration, in regards to comments at https://github.com/nu7hatch/gmail/pull/66#issuecomment-10631645 and https://github.com/nu7hatch/gmail/pull/66#issuecomment-13149719.

Cheers, 
Alex