Don't allow primitive types as the initial signal input

[saulshanabrook]

In cerebral, we only want to allow signals to be called with objects, not primitive types (cerebral/cerebral#252). If we wanted action tree to handle this for us, it should require an object as the initialPayload (and probably the subsequent payload). This is related to #2.

@Guria If this shouldn't be handled with action-tree can you reopen cerebral/cerebral#252 ?

[Guria]

@christianalfoni what your opinion here? should we allow only serializable objects in action-tree or keep it unopinionated about it?

[Guria]

Oh. I think we can restrict to Object, no matter if it serializable or not in action-tree and keep checking in cerbral dev-mode that payload is serializable

[saulshanabrook]

I opened this one just to make sure the type is Object, not touching on serializability.

[Guria]

Yeah. We should check it here, because we can't merge primitives.