Prevent leaking user's installed dictionary (Bug 22484)
Prevent leaking user's installed dictionary (Bug 22484)

Also set the Content-Language to "en-US" to prevent leaking the user's
default dictionary. Reported in https://bugs.torproject.org/22484 and
discussed in Section 4 of RFC 3282.  Thunderbird bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=1370217

infertux/torbirdy,infertux/torbirdy