Session improve

As discussed 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. So I check it first.

If the filed is not in user's properties, I just ignore 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 fixes some indent problem. You use tab instead of two spaces :) .