'==' behaves like '===' in ruby 1.9.

This fixes the bignum/case_compare_spec.rb rubyspec.

It seems however that this behaviour changed again in 2.0.
Compare:
```
1.9.3p362 :001 > b =  0x8000_0000_0000_0000 + 10
 => 9223372036854775818
1.9.3p362 :002 > b === b.to_f
 => true
1.9.3p362 :003 > puts RUBY_VERSION
1.9.3
 => nil
```

to

```
irb(main):001:0> b =  0x8000_0000_0000_0000 + 10
=> 9223372036854775818
irb(main):002:0> b === b.to_f
=> false
irb(main):003:0> puts RUBY_VERSION
2.0.0
=> nil
```
'===' differs from 'eql?' in ruby 1.9.
Bignum#to_s returns a US-ASCII encoded string.

This fixes bignum/to_s_spec.rb failures.
Merge pull request #661 from tychobrailleur/bignum_fix

Bignum failing specs