Proc.parameters return an empty array

Invoking the .parameters method of a Proc seems to return an empty array, rather than the parameters definition.

Repro steps:

``` ruby
p = Proc.new do |arg1: |
  puts arg1
end

puts "Params: #{p.parameters.inspect}"
```

Using ruby 2.2.0: (and it is the same for 2.1.x)

```
Params: [[:keyreq, :arg1]]
```

Using Jruby-head (ruby9000) as of today:

```
Params: []
```
