Remove implicit null skip from Hash to JSON serialization

[makenowjust]

Fixed #7052

I think Hash(String, String).from_str(%{{"foo": null}}) should be failed because String does not contain nil and JSON null is just nil. In fact Rust (serde) and Swift does not allow this.

Rust:

#[macro_use]
extern crate serde_derive;

extern crate serde;
extern crate serde_json;

use std::collections::HashMap;

fn main() {
    let _: HashMap<String, String> = serde_json::from_str(r#"{"foo": null}"#).unwrap();
}
// Error("invalid type: null, expected a string", line: 1, column: 12)

Swift:

import Foundation

let decoder: JSONDecoder = JSONDecoder()
let _: [String: String] = try! decoder.decode(Dictionary<String, String>.self, from: """
  {"foo": null}
""".data(using: .utf8)!)
// Swift.DecodingError.valueNotFound(Swift.String, Swift.DecodingError.Context(codingPath: [_DictionaryCodingKey(stringValue: "foo", intValue: nil)], debugDescription: "Expected String but found null value instead.", underlyingError: nil))

[straight-shoota]

Why not leave the spec expecting an exception?

[makenowjust]

@straight-shoota Good! Added.

[sdogruyol]

Thank you @makenowjust 👍