Add documentation to ActionDispatch::Request

I've added some documentation to the following:

- `ActionDispatch::Request#fullpath`
- `ActionDispatch::Request#original_url`
- `ActionDispatch::Request#media_type`

Could someone check that I've explained them correctly?

I also have a question about `Request#media_type` referring to it's documentation.

I created a simple blog and used `respond_with` so that the articles index responds to XML and JSON.

Here's what I get in the console:

```ruby
app.get '/articles.xml'
app.request.media_type
 => "application/x-www-form-urlencoded" 

app.get '/articles.json'
app.request.media_type
 => "application/x-www-form-urlencoded"
```

Should I be seeing `application/xml` and `application/json` respectively?