JRuby 9000: const_defined? does not support nested constants

``` console
$ ruby -v
jruby 9.0.0.0-SNAPSHOT (2.2.0p0) 2014-12-14 31f21b9 Java HotSpot(TM) 64-Bit Server VM 25.20-b23 on 1.8.0_20-b26 +jit [darwin-x86_64]
```

``` ruby
module Foo
  Bar = 42
end

# should return true but raises exception
Object.const_defined?("Foo::Bar")
```
