numerical order independently of the representation of the object.

Exchanged the order of enumeration of integer and string keys so
integer keys are first instead of string keys to better match
WebKit/JSC behavior.

Added test cases that document our enumeration order choice.
Review URL: http://codereview.chromium.org/75035

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1722 ce2b1a6d-e550-0410-aec6-3dcde31c8c00