feat(mailbox): send on unstarted will not return error

[vladopajic]

sending on unstated mailbox is not returning error anymore.

this was changed because if mailbox needed by actor is started after actor, Send() will result with error. in this situation it makes more sense for actor to block, since mailbox will get started soon.

example:

a := Combine(a1, a2, mbx)
a.Start()

in above example, actors a1 or a2 can potentially send data to mailbox mbx, but since it's not started it will result with en error. this situation is better for them if they are blocked until mailbox is not started.

[github-actions]

go-test-coverage report:

File coverage threshold (100%) satisfied:	PASS
Total coverage threshold (100%) satisfied:	PASS
Total test coverage: 100% (312/312)