This fixes the behaviour to match what synapse implements in practice.
If you use threepidCreds, you will just get an error about a missing
threepid_creds field. Synapse also treats this as an object. All clients
also seem to send threepid_creds, if they work on Synapse. Since
matrix.org requires an email currently for registration, most clients
that implement registration, will hit this issue.
a0f48ee89d/synapse/handlers/ui_auth/checkers.py (L145)fixes#3156fixes#2189
Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>
