[SPARK-2293] Replace RDD.zip usage by map with predict inside. - ASF JIRA

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In our guide, we use

val prediction = model.predict(test.map(_.features))
val predictionAndLabel = prediction.zip(test.map(_.label))

This is not efficient because test will be computed twice. We should change it to

val predictionAndLabel = test.map(p => (model.predict(p.features), p.label))

It is also nice to add a `predictWith` method to predictive models.

def predictWith[V](RDD[(Vector, V)]): RDD[(Double, V)]

But I'm not sure whether this is a good name. `predictWithValue`?