Validate field exclusion settings when EsSpark.saveJsonToEs() is executed

[jbaiera]

Users may save documents to Elasticsearch from a Spark RDD regularly or as raw JSON. In the scenario where they choose to save as raw JSON, explicit field inclusion and exclusions are ignored and the data is written as is. There should be a validation step when saving raw JSON to either throw or warn in the event that an explicit field exclusion or inclusion is present. The docs should also be updated to highlight this limitation.