expireAfter index option is not correctly encoded

Creating new indexes with expireAfter throws an error:

Example code:

*// Configure Mongo
let client = try MongoClient(Constants.mongo.url)
services.register(client)

let model = IndexModel(keys: ["createdAt": 1], options: IndexOptions(expireAfter: 10))
try client.db("auth").collection("keys").createIndex(model)*

Error:

*Fatal error: Error raised at top level: MongoSwift.ServerError.commandError(code: 197, message: "The field \'expireAfter\' is not valid for an index specification. Specification: { key:

, name: \"createdAt_1\", expireAfter: 10 }", errorLabels: nil): file /BuildRoot/Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-1001.8.63.13/swift/stdlib/public/core/ErrorType.swift, line 200***

The problem is that Mongo expects a parameter name expireAfterSeconds and MongoSwift uses expireAfter.

There are a simple solutions applying to IndexModel struct

1. Change expireAfter key to expireAfterSeconds
2. Add coding key string like expireAfter = "expireAfterSeconds"