mongodump/mongorestore error if source database has an invalid index option

Older driver versions allowed invalid index fields. When one of these invalid index fields exists, it gets dumped by mongodump and then upon mongorestore, the process encounters a "The field 'XXX' is not valid for an index specification" error.  For example:

2019-09-27T04:03:10.421-0700	Failed: testdb.food: error creating indexes for testdb.food: createIndex error: (InvalidIndexSpecificationOption) The field 'safe' is not valid for an index specification. Specification: { key: { a: 1.0 }, name: "a_1", ns: "testdb.food", safe: true }

Possible workarounds:

• Delete and re-create the index. Not a great option since dropping the index to re-create it could significantly affect performance.
• Perform the mongorestore with the "–noIndexRestore" option

Reproduction steps:

1. On a deployment running 3.2 or earlier, create an index with an invalid option using the mongo shell. A common invalid option previously introduced by one of the drivers was "safe".
2. Upgrade it to 3.4
3. Take a mongodump (any version, including 4.2.0)
4. Run mongorestore (any version, including 4.2.0)

Example of creating an index with an invalid option (on deployment running 3.2.23):

> db.foo.insert({a:1})

WriteResult({ "nInserted" : 1 })

> db.foo.drop()

true

> db.foo.insert({"a":1})

WriteResult({ "nInserted" : 1 })

> db.food.createIndex({"a":1},{"safe": true})

{

	"createdCollectionAutomatically" : true,

	"numIndexesBefore" : 1,

	"numIndexesAfter" : 2,

	"ok" : 1

}