Added/fixed Hash specs testing for private #hash methods

spec/ruby/core/hash/hash_spec.rb
  * Added spec that tests that a hash is generated for hashes containing
    values with private hash methods

spec/ruby/core/hash/merge_spec.rb
  * Fixed private key spec; a key's hash method is only called if
    a block is passed to merge/merge!
Fixed HAMT #merge & #hash to accept private #hash
Fixed HAMT #find_item to accept private #hash
Fixed Hash#hash to accept values w/ private #hash