pointless assertions so that bacon does not complaing

Hi @Sergey 

Thank you very much for this gem. I was having troubles since the rubymotion community didn't have mocks. 

This pull request is just to solve the messages like: 

```bash
Bacon::Error: empty specification: A method stub should ignore when the stubbed method is never called
	spec.rb:383:in `block in finish_spec': A method stub - should ignore when the stubbed method is never called
	spec.rb:403:in `execute_block'
	spec.rb:383:in `finish_spec'
	spec.rb:281:in `run_spec_block'
	spec.rb:294:in `run'

Bacon::Error: empty specification: A method stub with arguments should ignore when never called
	spec.rb:383:in `block in finish_spec': A method stub with arguments - should ignore when never called
	spec.rb:403:in `execute_block'
	spec.rb:383:in `finish_spec'
	spec.rb:281:in `run_spec_block'
	spec.rb:294:in `run'

Bacon::Error: empty specification: A method stub with arguments should ignore when called with expected argument
	spec.rb:383:in `block in finish_spec': A method stub with arguments - should ignore when called with expected argument
	spec.rb:403:in `execute_block'
	spec.rb:383:in `finish_spec'
	spec.rb:281:in `run_spec_block'
	spec.rb:294:in `run'

```