Alias the ActionDispatch::Request#uuid method to ActionDispatch::Request#request_id

When writing code, and wanting access to the `"X-Request-ID"` header, one uses the ActionDispatch::Request#uuid method. 

Generally, in actual code it looks like `request.uuid`. While I understand what this means, it isn't particularly clear. When one considers that it comes from the `"action_dispatch.request_id"` environment variable and that everything else I noticed in the Rails code corresponds to the convention in the following example, I think it serves to be a viable change. For instance, `request.remote_ip` corresponds to `"action_dispatch.remote_ip"`, and that is just one such example. 

Further, something that drives the point home to me is that `request.uuid` has the same tone as `person.string`, which returns the first name of that person. Sure, it is a string, but the method should be called `person.first_name` instead to be more descriptive. I am sure you agree with this.

After carefully examining the implementation of `config.log_tags = [:uuid]` which is closely related to the request_id uses in Rails, `config.log_tags = [:request_id]` will automatically begin working, because in the source code you have `request.send(tag)`, when `tag` is a symbol. By aliasing the method, this ensure that the functionality still works and further will continue to work for anyone who still opts to use ActionDispatch::Request#uuid instead of ActionDispatch::Request#request_id.

One last point: I searched through the Rails code and found no tests for the ActionDispatch::Request#uuid method. Due to the fact that I'm simply aliasing it and checked the implementation to make sure no changes had to be made for the log_tags code, I didn't include tests.