Use `be_an_instance_of` instead of `be_kind_of` on spme specs

This patch fixes some specs for the relationship between
instance and class.

And some of them related to rubyspec/rubyspec#198.
Fix a typo of String.new spec
Add Struct#select specs when not given a block
Update Struct#select when not given a block for 1.9
Merge pull request #2592 from kachick/struct-select-without_block_on_1.9

Struct#select should return an enumerator when not given a block on 1.9 and later
Update an Enumerator::Lazy#zip spec for MRI backported

Follow MRI-r42922