RFC7591: Use default values for 'response_types' and 'grant_types'

[azmeuk]

The RFC7591 dynamic client registration indicates default values for response_types and grant_types:

• If omitted, the default is that the client will use only the "code" response type.
• If omitted, the default behavior is that the client will use only the "authorization_code" Grant Type.

This is a better attempt than #509

What kind of change does this PR introduce? (check at least one)

• [x] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes

• [x] No

• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.

[azmeuk]

@lepture Can I help you go forward with this patch?

[azmeuk]

Thank you!

[lepture]

You're welcome.