Operators need to handle mapping to SPL reserved keywords

[cancilla]

If a JSON document contains keys that are reserved words in SPL, there is currently no way to map those key/value pairs into an SPL attribute.

For example, suppose I have the following JSON:

    {"type" : "a", "class" : "b"}

Since "type" and "class" are reserved words in SPL, you cannot have a schema that contains attributes with those names (i.e. tuple<rstring type, rstring class> is not valid).

The operators need to either handle the mapping of reserved key names (i.e. automatically map key "type" to something such as SPL attribute "type_") or allow the user to provide a custom mapping.

[markheger]

Operators should handle this similar to the XML parse operator with the ignorePrefix parameter

[schubon]

Closing after merge of Mark's changes.