Remove nil default value

[jpbetz]

Based on how we are observing the courier schema language used in practice. We plan to remove = nil defaults. What we’ve observed is that in many cases optional fields that could/should be marked as nil are not. This is inconvenient because it then requires that callees explicitly provide None for parameters that they should not need to. To fix this we are going to change the default behavior. All optional fields will be generated with = None by default in Scala code (= nil in Swift) unless a @explicit property is added to the field (exact name of the property TBD) .