Fix getting gists

[robrbecker]

Fixes #288
The type of the files when getting a gist isn't a list, it is a map. This is a breaking change. So anywhere you were accessing the files from a gist before, you'll need to update your code to expect a map.

Breaking change: In the Gist class, the old type of files was

List<GistFile>? files;

and the new type is

Map<String, GistFile>? files;

Breaking change: In the GistFile class, the name property is now filename

While I was in there I noticed there were a lot of files that don't generate the toJson method. It doesn't hurt to generate those so I updated all the places that weren't generating toJson.