GraphQl: Normalize id based on the origin resource

[soyuka]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	api-platform/api-platform#1045
License	MIT
Doc PR	na

When querying the following:

{
  language(id: "/api/languages/1") {
    id, foo
  }
}

where Language has an output it should get the following response:

{"data":{"language":{"id":"\api\/languages\1","foo":"bar"}}}

Where the id matches the origin resource IRI.

ping @phryneas

[phryneas]

@soyuka works fine for me for the id property - with another followup bug (sorry!):
querying _id should return 1, but returns /api/languages/1

[soyuka]

Many thanks @teohhanhui for the review !