Convert environment Links to json in Resource.to_json()

[MikeFE]

There appears to be a bug where Entry instances aren't converting the 'environment' sys field Link instance to json when calling to_json().

Hopefully this solution makes sense in general, it fixed the particular issue I was having.

Demonstration here.

[rubydog]

Hi @MikeFE, thank you for the contribution. Can you please add a test case for proposed changes?

Cheers