Fix confusion when beanstalkd doesn't contain body

When I put new job with body:
```
irb(main):011:0> job_body = "example"
irb(main):012:0> beaneater.tubes['default'].put(job_body)
=> {:status=>"INSERTED", :body=>nil, :id=>"113", :connection=>#<Beaneater::Connection host="localhost" port=11300>}
```
I received hash with ```:body=>nil``` and it confused me a bit because I sent body ```"example"```. Maybe better hide body key if answer from beanstalkd doesn't imply that?