[Spark 2060][SQL] Querying JSON Datasets with SQL and DSL in Spark SQL

JIRA: https://issues.apache.org/jira/browse/SPARK-2060
it would be more obvious what this does if you write it out, i.e.
```scala
val schemaData = if (sampleSchema.isDefined) json.sample(false, sampleSchema.get, 1) else json
```