Simplify implementation of consecutive return values.

There's no need to track extra instance variable state (e.g. @consecutive); 
instead, the implementation lambda can just do the right thing.

This also fixes #217. Previously, message_expectation got confused when it
was in consecutive mode (due to being stubbed with multiple return values)
and then got mocked with a single return value. This simplified implementation
fixes this bug.

@alindeman -- can you code review this, please?