OAuth2 RequestValidator method "save_bearer_token" should return the default redirect URI

[ehossack]

Is your feature request related to a problem? Please describe.
Oauth2 RequestValidator does not correctly implement superclass.

Per return type: https://github.com/oauthlib/oauthlib/blob/fe020db74199d5284c00d7735aa7d4ddc90f5d61/oauthlib/oauth2/rfc6749/request_validator.py#L357

the request validator should return the redirect URI, but the current method returns nothing:

django-oauth-toolkit/oauth2_provider/oauth2_validators.py

Line 645 in 3e11703

Describe the solution you'd like
The method should be altered to return the default redirect URI for the client

Describe alternatives you've considered
n/a

Additional context
n/a