Better username evaluation in get_user_obj

[wazy]

If Cognito was instantiated with an access token and without a username then get_user will return a user without a username property:

c = Cognito(user_pool, client_id, access_token)
u = c.get_user()
print(u.username) # prints None

This PR allows get_user_obj to check if username is not set and if so try to set it to self.username or the username from the metadata dictionary if it exists.

Order of preference for what username should be:

1. passed in username
2. self.username
3. username from the metadata

[wazy]

@ludeeus @pvizeli This is a bug I believe and this change should address it. Should I start opening issues to link to in PRs?