Allow the Dispatcher to set is_behind_proxy for Core::Request:

Currently, Core::Request's is_behind_proxy is set by Core::App
whenever a new Core::Context is provided (using a trigger, no less).

There are various problems with this:
* The App shouldn't care about is_behind_proxy
* The attribute shouldn't be set every time, since it's global
* Core::Context is unrelated to this

Instead, we made behind_proxy a global variable (as done by
GH #590) with its own trigger. Then it is set on instantiation by
Core::Dispatcher.

Then we can remove Core::App's _init_for_context method completely.

GH #590 approaches this issue in an entirely different and better
way, by suggesting we simply correct the environment before all
of this happens using a middleware.

For now we're just doing enough to untangle Core::Context more.