Correct JUnit notification for background steps

The background steps are copied in the ExecutionUnitRunner for each scenario in a feature with background. This is done to make them unique for each scenario, so that the JUnit notification later will not be ambiguous. When the background steps are executed, it is the original ones that are executed. If the background steps are not 'converted' from the original steps to the copies made earlier, when performing the actual JUnit notifications for an execution unit (scenario), then the background step appear as Unrooted test in the Eclipse JUnit tab:
![cucumber-junit-error](https://f.cloud.github.com/assets/2105308/1950127/4e0dd730-8136-11e3-9fd3-8012420fbe57.JPG)

This PR change the JUnitReporter to use the steps copied, and now stored, in the ExecutionUnitRunner, as base for the JUnit notification. Then the background steps appear correctly in the Eclipse JUnit tab:
![cucumber-junit-error-fixed](https://f.cloud.github.com/assets/2105308/1950132/aa68113a-8136-11e3-8a6e-2cfe3ef12d7f.JPG)

Fixes: #659.