patchbomb: don't unintentionally duplicate headers
This dict-like of headers is only mostly dict-like: if you set a key
that already exists, you it appends another value, rather than
replacing the one that was already present.
This fixes test-patchbomb-bookmark.t on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4229
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}