Container throws if injection registered for already instantiated type.

Prior to this commit, the container would happily allow you to register an
injection for a type that had already been instantiated. This led to
confusing-to-debug situations where an injection is registered but a
container-created instance doesn't have the injection applied.

Also covers factoryInjections.
[Bugfix beta] Allow setting of `undefined` value to a `content` property
[BUGFIX beta] Ensure context is unchanged when using keywords with itemController.

Prior to this change the following code would bind `this` in the
template block to the itemController's content (even though the keyword
form of `{{each}}` is used).

This change sets the child view's `_context` property to the current context when using
the keyword form of `{{each}}`, and a couple of confirming tests to
demonstrate using `itemController` specified in either the
`ArrayController` or the template directly.
Merge pull request #4645 from selvagsz/bugfix_allow_setting_undefined_to_props

[BUGFIX beta] Allow setting of `undefined` value to a `content` property
[BUGFIX beta] Fixes test suite failures relating to QUnitAdapter

Replace start() and window.start() with QUnit.start()
Replace stop() and window.stop() with QUnit.stop()
Remove start and stop from globals
[BUGFIX beta] use Object.keys when possible
Merge pull request #4676 from sly7-7/fix-ember-dot-keys

[BUGFIX beta] use Object.keys when possible
[BUGFIX beta] Update Backburner.
[BUGFIX beta] Ember.onerror now uses Backburner's error handler - #3401
license type
Merge pull request #4677 from rjackson/update-backburner

[BUGFIX beta] Upgrade Backburner, Switch Ember.onerror to Backburner's handler
Merge pull request #4679 from knownasilya/license

Add license type to package.json
Merge pull request #4669 from mfeckie/qunit-window-fix

[BUGFIX beta] Fixes test suite failures relating to QUnitAdapter
[DOC release] Observable addObserver and removeObserver return void.
Merge pull request #4682 from rjackson/rj-rebase-4484

[DOC release] Observable addObserver and removeObserver return void.
[BUGFIX beta] Do not rely on Array.prototype.map for logging version.
Merge pull request #4683 from rjackson/use-enumerable-utils-map-for-library-output

[BUGFIX beta] Do not rely on Array.prototype.map for logging version.
[BUGFIX beta] Add 'autocapitalize' and 'autocorrect' attributeBindings.

For Ios autocompletion disabling
Merge pull request #4685 from rjackson/rj-rebase-4407

[BUGFIX beta] Add 'autocapitalize' and 'autocorrect' attributeBindings.
[BUGFIX beta] Remove old reference to invoker.

As of 6a8137902da56b8b1060fcd0bdb748d5e7ae0092 we no longer use a custom
`invoker`, and the logic was moved into `registerObserver` directly.