Incorrect comment and small fix

The constructor comments of the OAuth1Request and OAuth2Request class had a <see cref="Parameters"/> element which could not be parsed. Replacing this with <see cref="Xamarin.Auth.Request.Parameters"/> solved this problem.

Furthermore, I found a problem when trying to compile the Xamarin.Auth class library. In the Dispose method response.Dispose (); was called but the compiled complained:
`'System.Net.HttpWebResponse' does not contain a definition for 'Dispose' and no extension method 'Dispose' accepting a first argument of type 'System.Net.HttpWebResponse' could be found`
Replacing the call from Dispose() to Close() (which internally will call Dispose()) solves this problem.