Hash#== o raises exceptions unless o has to_hash method

```
% ruby -e 'p( {} == true )'
false

% bin/mruby -e 'p( {} == true )'
trace:
        [1] mrblib/hash.rb:15:in Hash.==
        [0] -e:1
mrblib/hash.rb:15: undefined method 'to_hash' for true (NoMethodError)
```
