Only throw an initialValue error if it is null or undefined (i.e. not 0 or some other falsy value)
Merge pull request #3327 from matthooks/master

Allow reduceComputed to have an initial value of 0.
Add shortcut for whitelisting all modifier keys on {{action}}
Modifier key-independent action helper use cases can be less verbose and more future-proof.
fixing grammer in computed property docs
Merge pull request #3335 from GavinJoyce/master

fixing grammer in computed property docs
Merge pull request #3323 from codefox421/action-modkeys-any

Add shortcut to action helper for whitelisting all modifier keys