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]]
```
Merge pull request #438 from rspec/fix_jruby

Work around JRuby returning empty parameters on repeated calls.