Work around JRuby returning empty parameters on repeated calls.

Created by: JonRowe

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

e.g. on JRuby 1.7.4

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

And with this "fix"

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