This is due to a recent change made in #99

I'm relatively new to AppAuth and GTMAppAuth, so it's very possible I'm misunderstanding how something should be working here - but here's what I'm seeing.

As part of #99 - if you now attempt to init an OIDAuthorizationRequest with a response type of token, an exception is thrown. The problem is that GMTOAuth2 apparently used to use token as a response type, therefore a couple (related) things now break:

1. An exception is thrown when merely deserializing a stored OIDAuthorizationRequest that was set to token as the response type

2. https://github.com/google/GTMAppAuth - in the GTMOAuth2KeychainCompatibility.m, when they are deserializing the old keychain entry to migrate it to a new one, they explicitly use token as the response type in authorizeFromPersistenceString:serviceProvider:tokenURL:redirectURI:clientID:clientSecret:.

3. Even after I do the migration (by commenting out the change in Throws error on unsupported response type #99), the problem seems to be that OIDAuthState stores a request/response history, so _lastAuthorizationResponse is set to be a token response. So, if I make the changes for Throws error on unsupported response type #99 live again, it will still throw the exception every time it grabs the new authorization object that is stored in the keychain.

So, should the changes in #99 be modified to not crash when deserializing an OIDAuthorizationRequest or is there some other way to handle this?