sasl: Re-introduced XOAUTH2 in the default enabled authentication mechanism

Following the fix in commit d6d58dd5 it is necessary to re-introduce
XOAUTH2 in the default enabled authentication mechanism, which was
removed in commit 7b2012f2, otherwise users will have to specify
AUTH=XOAUTH2 in the URL.

Note: OAuth 2.0 will only be used when the bearer is specified.