Before, you could not POST your credentials to a password-connector's
endpoint without GETing that endpoint first. While this makes sense for
browser clients; automated interactions with Dex don't need to look at
the password form to fill it in.

A symptom of that missing GET was that the POST succeeded (!) with

    login successful: connector "", username="admin", email="admin@example.com", groups=[]

Note the connector "". A subsequent call to finalizeLogin would then
fail with

    connector with ID "" not found: failed to get connector object from storage: not found

Now, the connector ID of an auth request will be updated for both GETs
and POSTs.

Signed-off-by: Stephan Renatus <srenatus@chef.io>