[swift 4] Default value for optional init params

Hi.

Summary:

It would be really nice if Swift model objects had a default nil value for optional parameters so for model objects with many optional parameters we can create them more simply. There's a similar issue on the swagger-codegen repo https://github.com/swagger-api/swagger-codegen/issues/8365. This is a trivial, backwards compatible change to the swift handlebars template.

Details:

Currently the template generates code like this:

public struct Order: Codable {

    public var _id: Int64
    public var petId: Int64?

    public init(_id: Int64, petId: Int64?) { 
        self._id = _id
        self.petId = petId
    }
}

It would be really nice if it have default values to the optional init methods, like this:

public init(_id: Int64, petId: Int64? = nil) { 

This seems a trivial change in the template so any objections to a pull request?

Start by locating the Swift 4 Handlebars template that generates model initializers. Update the optional-parameter handling so optional init parameters default to nil, then verify that generated Swift models use the requested initializer form without changing required parameters.