Add under-tested Content-Disposition.

--HG--
extra : rebase_source : cb4d397901e4dfb2ac1cc284d3c8e4edd9bdc65a
Import all of the changes from master onto the new jsmime version.

--HG--
extra : rebase_source : bb64dc040e33e69028322e3928701d28be5a56c3