Fix session security problem while login

As mentioned in [pull request 184](https://github.com/deployd/deployd/pull/184), the improve of session has a security hole. 

It is reasonable that the field you want to save in session should be in user's
properties. This commit checks it.

If the filed is not in user's properties, I just ignore it and don't throw any error. I'm not very familiar with deployd, so I don't know if I'm doing it right.

This commit also fix some indent problem.