"null" value is deserialized to null instead to "null"

[michibe]

A JSON in the form {"propertyName":"null"} is interpreted as the propertyName is null e.g. {"propertyName":null}.

This makes it impossible to send a "null" value as String within JSON.

To Reproduce
Json.parseToJsonElement("""{"propertyName":"null"}""")

Expected behavior
A JSON in the form {"propertyName":"null"} is deserialized as a String with value "null"

Environment

• Kotlin version: 1.5.0
• Kotlin platforms: JVM, JS