Fix of ```test.testLogging.showStandardStreams = false```

The setter was ignoring the flag, so it wasn't possible
to disable standardStreams using this setter.

The following sample easily demonstrates the problem with Gradle 2.2.1+:

```groovy
test {
    testLogging {
        showStandardStreams = false
    }

    // this prints true no matter what value you actually will use in assignment
    println("In-tests logging configured: ${project.test.testLogging.showStandardStreams}")
}
```