Revert "Be able to change the accessToken of a request" - #80

More specifically:

• For clients registered as public clients ("native client app" in the Azure portal), the Resource Owner Password Credentials flow is supported, but does not use a secret.
• For clients registered as confidential clients ("web app/web API" in the Azure portal), the Resource Owner Password Credentials flow is not support (even though it technically works, when you include a secret).

For the tests here, the client (i.e. identified by CLIENT_ID) should be registered as a native client app in Azure AD, and no client_secret parameter should be used.