prevent quoted nulls ("null") when to_json is called on nil relationship

[soopa]

Fixes the case where calling something like:

resource.to_json(:relationships => { :nested_resource => { :only => [:id] } })

When the nested resource returns nil, would return invalid JSON like…

{
  "id":1,
  "nested_resouce":"null"
}

Rather than valid JSON like…

{
  "id":1,
  "nested_resouce":null
}

(Note the quotes around null)

[soopa]

Acceptable?

[soopa]

Checking in, 2 years later. I've just rebased this cleanly on latest master and the build is passing.

@solnic @dkubb could one of your merge this please? I'd love to get the reference to my fork out of my Gemfile and resolve this issue for anyone else who may be stumbling upon it.

Thanks!

[soopa]

Updated branch target in #41, closing this one.