Fixes #239.

The missing charset in the Content-Type header caused train 124 to fail. I could have patched it in 124 but it feels like we're close enough to train 125 to patch it there instead now (and I don't even know if there's ops bandwidth to test it out in 124 again anyway).

I've also added the Sender header, although I don't think its absence was causing problems. The spec seems pretty clear that it falls back to the value of From by default, but email servers are a capricious species so better to be explicit.

The Content-Language header can only be patched in the auth server (patch incoming). The Content-Transfer-Encoding header is by design.

@mozilla/fxa-devs r?