Refactoring validate_missing_parameters to validate_required_parameters.

Inside of assign_attributes, there is a method which checks to make sure that particular parameters exist. The previous method name was ``validate_missing_parameters!`` and sounds like some parameters should be missing, when in fact it's meant to check to make sure none of the parameters passed in as arguments are missing.

Because of this, I think that ``validate_required_parameters!`` is a more apt name. I also removed a space in the middle of an empty hash.