Fix "IsReference/GetReference" for all types in components

[vkd]

I've found that methods IsReference() and GetReference() don't work as expected for all component's types except schemas.

Affected version: v0.21.8 d3dcac4e5f6f13b0c3afa736e0d449a9abe9c811.

Code example:

import (
	"github.com/pb33f/libopenapi"
	v3 "github.com/pb33f/libopenapi/datamodel/high/v3"
)

func xxx() {
	doc, _ := libopenapi.NewDocument(specRaw)
	v3Model, _ := doc.BuildV3Model()
	
	var components *v3.Components = v3Model.Model.Components
	for _, v := range components.Responses.FromOldest() {
		v.GoLow().IsReference()  // false for all objects
		v.GoLow().GetReference() // "" for all objects
	}
}

Configuration example:

components:
  responses:
    A1:
      description: "a1"
      content:
        application/json:
          schema:
            type: object
    A2:
      $ref: "#/components/responses/A1"
    A3:
      $ref: "#/components/responses/A2"

Questions:

• datamodel/low/v3/components.go: is there anything missing or what exactly that code is doing? All unit tests are passed, but maybe I miss something.
• datamodel/low/v3/create_document_test.go: should I adjust this test check into something else?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.61%. Comparing base (d3dcac4) to head (c5d5771).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #389      +/-   ##
==========================================
+ Coverage   99.55%   99.61%   +0.06%     
==========================================
  Files         165      165              
  Lines       21606    21621      +15     
==========================================
+ Hits        21509    21537      +28     
+ Misses         92       79      -13     
  Partials        5        5              

Flag	Coverage Δ
unittests	99.61% <100.00%> (+0.06%)	⬆️

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