Bug 978005 - [email] Fix usage of Bareword DOM constructors TextDecoder ...

...and TextEncoder. r=asuth

land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/288

Backporting this commit from master to fix the `JavaScriptError: (17) TypeError: Constructor TextEncoder requires 'new'`error seen, eg, in this Travis run: https://travis-ci.org/mozilla-b2g/gaia/jobs/21196584

Note, I'm still working on reproing that this definitely fixes the tests locally for me. Maybe Travis will get there first :-P