Support marking a link as part of an array

[chrissimon-au]

This permits defining a link as part of an array - even if there is only a single link with that rel name. Partially this was done to support following the recommendation from http://stateless.co/hal_specification.html:

Note:
If you're unsure whether the link should be singular, assume it will be multiple. If you pick singular and find you need to change it, you will need to create a new link relation or face breaking existing clients.

In particular, we intend to use this for adding curies to the _links array, as all examples of curies show them as an array of links, even when there is only a single link, e.g.

"_links": {
  "curies": [
    {
      "name": "doc",
      "href": "http://haltalk.herokuapp.com/docs/{rel}",
      "templated": true
    }
  ],

  "doc:latest-posts": {
    "href": "/posts/latest"
  }
}

[samuelabj]

Thanks @chrissimon-au. Would you mind adding tests to confirm a single rel without IsRelArray still renders a single link and singular rel with IsRelArray renders an array?

[chrissimon-au]

No problem - apologies for not including them in the first place!

The case where a single rel without isRelArray renders a single link is covered by existing test cases - the first two models in GetLinkTestModels.

Have updated the PR to add a unit test to confirm that isRelArray is stored properly on Link, and two more test cases to GetLinkTestModels:

1. to confirm that isRelArray:true will cause an array of a single link
2. to confirm that isRelArray:true on a link where there is more than one with the same rel will not impact on it being rendered as an array

Let me know if you'd like more tests.

[samuelabj]

Forgot to say thanks for adding the tests!