Add assertions Regexp#===(obj) in test_grep

Array#grep(obj) invokes `pattern === element` for each element.

Regexp#===(obj)

- obj is kind of String, Symbol -> matches Regexp
- obj responds to :to_str -> matches Regexp
- obj does not respond to :to_str -> returns false

OSX::NSNumber does not respond to :to_str, then Regexp#===(nsnum)
should returns false. but Regexp#===(nsnum) on ruby-2.x invoked
OSX::NSNumber#to_str and raises an exception.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2557 f4f36d40-9719-0410-9868-dde021a23eda