Fixed isWritable for badly formed SpEL expressions

Before this change isWritable() could return true
for a badly formed expression. That is because the
decision about whether something is writable was made
based on the node type rather than whether the node
represented something that could actually be resolved
to be a real thing. This change ensures a resolution
check is done and isWritable() should only return
true if a subsequent setValue() will succeed.

Issue: SPR-10610