MessageExtras asJsonObject() should return a copy

[QuintinWillison]

JsonObject instances are mutable. See this review comment.

I didn't want to knee jerk make this change without due consideration at the point of releasing 1.2.1 so this needs careful consideration as there may be other areas in the codebase where we're using Gson in a similar manner and should be addressing this concern.

┆Issue is synchronized with this Jira Task by Unito