Issue Description:

The following code generates different output on macOS and Linux. I am using Swift 4.2.1

import Foundation
struct JsonShit: Codable {
    let whatTheFuck: String
}
let shit = JsonShit(whatTheFuck: "???")
let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase
try print(String(data: encoder.encode(shit), encoding: .utf8))

The output generated on macOS:

"Optional("{"what_the_fuck":"???"}")"

The output generated on Linux:

"Optional("{"whatTheFuck":"???"}")"