Bug 874405 - [email/IMAP] snippet/partial body fetch breaks on zero-length bodies. r=jrburke

landing https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/206
Merge pull request #9914 from asutherland/email-zero-length-bodies

Bug 874405 - [email/IMAP] snippet/partial body fetch breaks on zero-length bodies. r=jrburke
tests only: fixing an error when thread_ui_test.js runs on its own due to lack of mock. a=npotb