resource.canWrite() should use the agent configured on the store

[Polleps]

If you set up a store with an agent, fetch a resource, then check resource.canWrite() it will return false because you did not pass an agent. But when you then edit the resource and save it it will succeed since it used the agent set in the store to sign the commit.
I think the user expects resource.canWrite() to use the store agent because other methods do this.

Also the docs suggest this is the case, I think this happened because I changed useCanWrite to work like this but not resource.canWrite