Add ability to allow method calls on RepresentedObjects

[orbanbotond]

Add the ability to allow a call on 'represented object', like

expect(subject).to represent(:me) do |represented|
  allow(represented).to receive(:get_authentication_token)
end

In order to support this scenario:

expose :token do |user, options|
  user.get_authentication_token.token
end

[mshappe]

+1

Without this, honestly, I can't use these matchers.