module Rspec::Core::InstanceExec

def instance_exec(*args, &block)

- only necessary for ruby 1.8.6
only place we need it
- this keeps it scoped to this class only, which is the
InstanceExecHelper module
- uses singleton_class of matcher instead of global
http://eigenclass.org/hiki/bounded+space+instance_exec
based on Bounded Spec InstanceExec (Mauricio Fernandez) 
def instance_exec(*args, &block)
  singleton_class = (class << self; self; end)
  begin
    orig_critical, Thread.critical = Thread.critical, true
    n = 0
    n += 1 while respond_to?(method_name="__instance_exec#{n}")
    singleton_class.module_eval{ define_method(:__instance_exec, &block) }
  ensure
    Thread.critical = orig_critical
  end
  begin
    return send(:__instance_exec, *args)
  ensure
    singleton_class.module_eval{ remove_method(:__instance_exec) } rescue nil
  end
end