phabricator: convert unicode to binary when writing patches
This is a quick fix to make `hg phabread D189` work.
It seems we might want to replace all `r''` to `u''`, and add more
`encoding.*to*` to be more explicit when interacting with `json` module.
Differential Revision: https://phab.mercurial-scm.org/D192
(binary:application/octet-stream)