Bug 978005 - [email] Fix usage of Bareword DOM constructors TextDecoder and TextEncoder. r=asuth

Fix our node-buffer shim and land changes from
https://github.com/mozilla-b2g/jswbxml/pull/9 which in turn resulted in
https://github.com/mozilla-b2g/jsas/pull/16

(landing steps being done by asuth)
Merge pull request #288 from asutherland/fix-bare

Bug 978005 - [email] Fix usage of Bareword DOM constructors TextDecoder and TextEncoder. r=asuth