redirectOnError does not work

[judgej]

With reference to issue #8 cancelling an authorisation from the Xero authorisation flow should return the user to the application, but instead takes the user to the Xero organisation dashboard.

The redirectOnError parameter when fetching the temporary token should ensure the user correctly returns, but is not working as expected.

[judgej]

It looks like it is used on the authorise URL and not the temporary token URL:

https://github.com/XeroAPI/Xero-Net/blob/master/Xero.Api.Example.Applications/TokenStoreAuthenticator.cs#L94

[judgej]

Fix made.

Undocumented, when the user returns from a cancellation, the user returns with an error + error_description pair of parameters, instead of oauth_problem + oauth_problem_advise. This needs a little more thought and some documentation on how to handle it consistently.