New PUT route returns null source parameter

[JoshBrodieNZ]

Hey team,

Started to put together a PR and realised I couldn't determine what the intended behaviour is here.

I note that the API's new update_parser takes only a target argument. However, the PUT route is marshalled with resource_fields which contains a source argument also. As a result, the PUT request is responded to with a null source, such that submitting {"target":"127.0.0.1"} to /api/routes/test.com results in a response containing {"source": null,"target": "127.0.0.1"}.

Should this return value draw the source from a valid location, or should source be omitted from the return value?