Fixed description of Hash#replace on frozen instance specs.

Fixed to match actual assertions made: from "raises a RuntimeError" to
"raises a TypeError" on pre-1.9 specs.
Updated spec/core/hash with specs from master
Removed ruby 1.9+ specs in ruby/core/hash
Added Hash specs testing for private #hash methods

The spec added in spec/ruby/optional/capi/hash_spec.rb tests that
Hash#find_item can find items that have keys with private hashes, as
Hash#find_item does not have specs in ruby/core/hash.
Fixed Hash to work with keys with private #hash

Hash#hash and Hash#rehash fixed to work with hash keys with
private #hash methods.