Appease req.setHeader by passing bytes, not ints.

Appease req.setHeader by passing bytes, not ints.

twisted.web.http.Request.setHeader() really wants a "bytes" object, but
we've been passing integers like len(body). Twisted-12.3 started to
complain about this (with a DeprecationWarning), but the warning is
usually silenced because py2.7 disables deprecations by default.

This fixes Tahoe's misbehavior, but others remain (in Nevow, at least).
I plan to set up some tooling to run tests with
PYTHONWARNINGS=default::DeprecationWarning and collect others. We won't
be able to fix the ones that occur outside of Tahoe, but at least we
should be able to fix our own.

refs ticket:2312