WebApiOpenApiDocumentGenerator: Support multiple Route-attributes on Controller level

[andreasbalzer]

This PR adds support for multiple Route-attributes on controller-level. For example, an API could respond both as api/beverages and api/fluids.

        [System.Web.Http.Route("api/{regionId:int}/beverages")]
        [System.Web.Http.Route("api/{regionId:int}/fluids")]
        public class BeveragesController : RegionalItemController
        {
            [System.Web.Http.HttpGet]
            public string[] Get(int regionId)
            {
                throw new NotImplementedException();
            }
        }

Observed behavior: WebApiOpenApiDocumentGenerator threw an exception, as it only allowed one Route-attribute on controller level

Expected behavior: Two paths get created.

Issue addressed partly: 1354 NSwag.MSBuild does not support API versioning or multiple routes

[andreasbalzer]

@RicoSuter please let me know if you like to see changes or if there is any way I can help.

[RicoSuter]

As I do not actively support WebAPI anymore I'll just assume this is fine.
Thanks for the PR.