JSON Serialization of IMap Keys

[TimWhiting]

Because this uses toJsonK and fromJsonK, non-String keys don't automatically get converted to Strings in the toJson map output, and also don't get converted back from Strings in fromJson.

PR #25 is a temporary solution until json_serializable can detect and handle these cases.

Here is the json_serializable issue
google/json_serializable.dart#396

[marcglasberg]

Fixed in version 7.0.6-dev0

Thanks for the PR!