Make ROPG flow to require client auth as stated in the RFC

[nbulaj]

Require client authentication for Resource Owner Password Grant as stated in OAuth RFC

[IMPORTANT] you need to create a new OAuth client (Doorkeeper::Application) if yoo didn't
have it before and use client credentials in HTTP Basic auth if you previously used this grant
flow without client authentication. For migration purposes you could enable
skip_client_authentication_for_password_grant configuration option to true, but such behavior
(as well as configuration option) would be completely removed in a future version of Doorkeeper.
All the users of your provider application now need to include client credentials when they use
this grant flow.

Fixes #1412 (comment)

[ghiculescu]

@nbulaj should https://github.com/doorkeeper-gem/doorkeeper/wiki/Using-Resource-Owner-Password-Credentials-flow be updated? The current example doesn't mention this at all.