1. Unsealed the Http*Attributes so that they can be extended and customized.
2. Added the same constructors as HttpGet to the rest of the Http*Attributes.
3. Added unit tests to validate the implementations for the IActionHttpMethodProvider.
4. Added functional tests to cover extra attribute routing scenarios like a test for an
action with an HttpDeleteAttribute on it and action with AcceptVerbsAttribute and an
action with a custom HttpMergeAttribute implemented.
This change allows a user to override a route prefix set using
[Route("...")] on the controller by providing a route template
on the action that starts with "~/" or "/". For example,
[HttpGet("~/...")] or [HttpGet("/...")]
If the user specifies a template in [Route] that starts with "~/"
or "/", we will just strip the prefix from the template and use
the remaining part of the template.
The reason to do this is that there's a reasonable extensibility
scenario where a user can implement a global prefix for routes as
a convention (using IReflectedApplicationModelConvention), and use
~/ to escape that prefix (just like we support with action-level routes).
Javier Calvarro Nelson
Ryan Nowak