Hash#hash fails when keys & values are the same

Using jruby-1.7.9 in 1.9-mode built from ruby-build on OSX.

``` ruby
{:foo => :foo}.hash                # => 1
{:bar => :bar}.hash                # => 1
{:foo => :foo, :bar => :bar}.hash  # => 2
{:baz => :baz, :qux => :qux}.hash  # => 2
```
