merge can create partial Immutables

[raorao]

When merging an immutable object with a mutable object, the resulting immutable object can contain mutable objects, but still identifies as immutable. For example, here's current behavior:

obj = Immutable( { key: 'value', collection: [ 1,2,3 ] } )
Immutable.isImmutable( obj ) //true
Immutable.isImmutable( obj['collection'] ) //true

newObj = obj.merge( { collection: [ 1,2,3 ] } )
Immutable.isImmutable( newObj ) //true
Immutable.isImmutable( newObj[ 'collection' ] ) //false
Immutable.isImmutable( newObj[ 'collection' ].push ) //danger!

its unclear what expected behavior should be. Personally, I think .merge should deeply convert everything that gets passed in to be immutable, as these partially immutable objects can create strange bugs. But maybe we should give users more flexibility. We could just add an isDeeplyImmutable function, and let users deal with the consequences.

[rtfeldman]

Yep, it should be deeply immutable. This is a bug. The guarantees of immutability are important to maintain...I'll fix this in a jiffy. Thanks!