Bug 864867 - Keep track of partially fetched bytes now that we attempt larger or complete download of bodies r=asuth.

landing lightsofapollo's patch from
https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/191
Merge pull request #9471 from asutherland/email-4k-fix

Bug 864867 - Keep track of partially fetched bytes now that we attempt larger or complete download of bodies r=asuth.