Work around JRuby returning empty parameters on repeated calls.

This fixes a JRuby bug where by repeated calls to parameters on unbound 
instance methods falsely return []

e.g. on JRuby 1.7.4
```Ruby
 String.instance_method(:replace).parameters
 # => [[:req]]
 String.instance_method(:replace).parameters
 # => []
```

And with this "fix"

```Ruby
 String.dup.instance_method(:replace).parameters
 # => [[:req]]
 String.dup.instance_method(:replace).parameters
 # => [[:req]]
```