Attribute Routing: Implement `~/` for overriding a prefix

[rynowak]

An attribute route starting with ~/ overrides any value set on the controller with [Route(...)].

Need to modify the code that joins templates + add tests.

[javiercn]

RouteAttribute on Controller	HttpGet on action	Result
[Route("api")]	[HttpGet("~/override")]	"override"
[Route("~/api")]	[HttpGet("~/override")]	"override"
[Route("/")]	[HttpGet("~/override")]	"override"
[Route("")]	[HttpGet("~/override")]	"override"
null	[HttpGet("~/override")]	"override" (null meaning no [Route(...)] on the controller
[Route("api")]	[HttpGet("/override")]	"override"
[Route("~/api")]	[HttpGet("/override")]	"override"
[Route("/")]	[HttpGet("/override")]	"override"
[Route("")]	[HttpGet("/override")]	"override"
null	[HttpGet("/override")]	"override" (null meaning no [Route(...)] on the controller

I've used HttpGet as a sample, same applies to other verbs. I'm going to start by implementing everything as override and making AttributeRouteTemplate.Combine strip the "" from the left portion of the route, we can discuss if we want to fail early and throw an exception if we see "" on the template for RouteAttribute

Also we need to figure out how "~/" plays with areas. Whether it should override the area or not.

@rynowak Thoughts?

[javiercn]

Discussed offline: No need to think of areas for this.

[rynowak]

Still good question about [Route("~/...")] - my 2c is to be permissive here. Seems silly to throw when we kinda know what you mean already.

Plus there's a reasonable extensibility scenario here. I'm a user, and I implement a global prefix for routes as a convention (using IReflectedApplicationModelConvention). I can use ~/ to escape that prefix (just like we support with action-level routes).

[javiercn]

As per discussion offline, we are making "/" and "~/" will expose the same behavior (both will override the value in [Route(...)] The semantics on the table above will be the same for both values

[javiercn]

@yishaigalatzer @rynowak take a look at the PR