docker login dict return missing data if configuration is passed as params without a ~/.docker/config.json config file

Current status:

Running the docker-py low level API (didn't check the high level API) login commands returns the following JSON when it's given a valid user, pass & host as Parameters (as in code params) without a ~/.docker/config.json file existing will return the following:

{u'IdentityToken': u'', u'Status': u'Login Succeeded'}

Running the same command with a ~/.docker/config.json file will return the following:

{'username': u'myuser', 'password': u'mypass', 'email': None, 'serveraddress': u'https://index.docker.io/v1/'}

Regardless of the return the login seems to work so this bug seems limited to the returned dict.

Expected status:

The dict values returned should be the same regardless of if you passed the login function the user\pass\host as params or had them taken from a file, making them empty in one case and not the other makes it seems like something is wrong with the auth.

I would also like to have some more celerity in the documentation as to what happens if both are passed (is it a combination of the two or does one take priority and the other is ignored? if so which one?)

System stats

docker==3.5.1
docker-pycreds==0.3.0
Python 2.7.15
Client:
Version: 18.09.0
API version: 1.39
Go version: go1.10.4
Git commit: 4d60db4
Built: Wed Nov 7 00:48:47 2018
OS/Arch: linux/amd64
Experimental: false

Server: Docker Engine - Community
Engine:
Version: 18.09.0
API version: 1.39 (minimum version 1.12)
Go version: go1.10.4
Git commit: 4d60db4
Built: Wed Nov 7 00:19:08 2018
OS/Arch: linux/amd64
Experimental: false

Start with the low-level DaemonApiMixin.login API and compare its returned dict when credentials come from parameters versus ~/.docker/config.json. Verify the behavior on the reported docker 3.5.1 and Python 2.7 setup, then document or test the priority when both sources are supplied and make the returned values consistent.