Hash cleaned when used as named parameters (JRUBY_OPTS=--2.0)

Hi,

I did try jruby-1.7.10,
- MacOS  
- Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609)

Some of my project spec's did fail and with jirb (export JRUBY_OPTS=--2.0) I could narrow it down to this example

``` ruby

def foo(bar: nil)
end

a = {bar: 1}

foo a

a.inspect # output {}

```

jruby clears the hash. MRI does not clear the hash.
