Also makes func.caller return null if the caller is a bound function,
matching JSC.
Fix bug preventing poisoned setters from triggering.

TEST=mjsunit/function-bind, mjsunit/strict-mode

Review URL: http://codereview.chromium.org/8333019

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9681 ce2b1a6d-e550-0410-aec6-3dcde31c8c00