Exploit from_iterable for Contexts with no __dict__

As @nvie pointed out to me, my earlier patch "breaks for people who are using Jingo/Jinja2 templates, since their Context has no attribute __dict__."

So after a couple of suggestions, we realized, and I've tested this locally too, that this new approach works well (for probably all applicable cases).
Updating CHANGELOG and CONTRIBUTORS
Bumping version to 1.2.8