* hash.c (rb_hash_assoc): performance improvement by replacing
  compare function in RHASH(hash)->ntbl->type temporarily like r42224.
  it falls back to rb_hash_foreach() if st_lookup() doesn't find the key.

* test/ruby/test_hash.rb: add a test for above.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42237 b2dd03c8-39d4-4d8f-98ff-823fe69b080e