Revert "Revert "Merge pull request #754 from ReactiveCocoa/short-switchToLatest""

This reverts commit 204ae0a40152acdeda03bdd00401152796d00ebc.
Add failing test verifying that -switchToLatest is cold
Refactor -switchToLatest to actually be cold
Save both disposables
Merge pull request #757 from ReactiveCocoa/short-switch-redux

Reimplemented -switchToLatest redux