fix exit code with Fiber.yield trick

When run any rspec examples wrapped with em-spec, then exit code is always 1. Example: 
    $ rspec
    Run options: include {:focus=>true}
    
    All examples were filtered out; ignoring {:focus=>true}
    ..
    
    Finished in 0.00114 seconds
    2 examples, 0 failures
    
    Randomized with seed 15661
    
    $ echo $?
    1
This patch allow to respectively shut down the reactor (with calling shutdown hooks if necessary) and return proper return code.