Created Unassigned: Improve experience of using UriPathExtensionMapping with AttributeRouting [1086]

__Scenario__:
Its popular with users to use Uri path extensions to get content in a particular format even though one could use Accept headers. My understanding is that this is for users from browsers who cannot modify headers of the request that a browser sends.

I have the following ValuesController which uses AttributeRouting.

```
[RoutePrefix("api/values")]
public class ValuesController : ApiController
{
[HttpGet("")]
public IEnumerable<string> GetAll()
{
return new string[] { "value1", "value2" };
}

[HttpGet("{id}")]
public string GetSingle(int id)
{
return "value";
}

[HttpPost("")]
public void Post([FromBody]string value)
{
}

[HttpPut("{id}")]
public void Put(int id, [FromBody]string value)
{
}

[HttpDelete("{id}")]
public void Delete(int id)
{
}
}
```

Now I would like to use UriPathExtensionMapping for the GetAll and GetSingle actions. I would like users to do "/api/values.xml" and "/api/values/10.xml" etc.

__Issue__:
I cannot do something like below for GetAll action, because we append the RoutePrefix with the action's RouteTemplate (ex: api/values/.{ext}). As you can notice this is not the desired behavior.

```
[HttpGet("")]
[HttpGet(".{ext}")]
public IEnumerable<string> GetAll()
```

__Workaround__:

__NOTE__: Here I would be placing the controller name on each of the individual action's route template, which ideally I would like to avoid.

```
[RoutePrefix("api")]
public class ValuesController : ApiController
{
[HttpGet("values")]
[HttpGet("values.{ext}")]
public IEnumerable<string> GetAll()
{
return new string[] { "value1", "value2" };
}

[HttpGet("values/{id}")]
[HttpGet("values/{id}.{ext}")]
public string GetSingle(int id)
{
return "value";
}

[HttpPost("values")]
public void Post([FromBody]string value)
{
}

[HttpPut("values/{id}")]
public void Put(int id, [FromBody]string value)
{
}

[HttpDelete("values/{id}")]
public void Delete(int id)
{
}
}
```

__Proposal__:
Could we introduce ability for individual actions' route attribute to ignore the RoutePrefix.

Example(__Notice__ the GetAll action's attribute):
```
[RoutePrefix("api/values")]
public class ValuesController : ApiController
{
[HttpGet("")]
[HttpGet("~/api/values.{ext}")] //NOTE
public IEnumerable<string> GetAll()
{
return new string[] { "value1", "value2" };
}

[HttpGet("{id}")]
[HttpGet("{id}.{ext}")]
public string GetSingle(int id)
{
return "value";
}

[HttpPost("")]
public void Post([FromBody]string value)
{
}

[HttpPut("{id}")]
public void Put(int id, [FromBody]string value)
{
}

[HttpDelete("{id}")]
public void Delete(int id)
{
}
}
```