Add an optionalFrom to create optional array of RawRepresentables

[marcelofabri]

#61 introduced a function to create an array of RawRepresentables.

However, we have a model that contains an optional array of enums, so optionalFrom would be needed. An implementation would be:

@warn_unused_result
public func optionalFrom<T: RawRepresentable where T.RawValue: Convertible,
    T.RawValue == T.RawValue.ConvertedType>(field: String, defaultValue: T? = nil) -> [T]?
{
    let value = try? self.JSONFromField(field)
    guard let array = value as? [AnyObject] else {
        return nil
    }

    let rawValues = try? array.map { try T.RawValue.fromMap($0) }
    return rawValues?.flatMap { T(rawValue: $0) ?? defaultValue }
}

If it's a welcome addition, I can make a PR adding it and some tests 🚀

[marcelofabri]

Actually, just this would be enough:

public func optionalFrom<T: RawRepresentable where T.RawValue: Convertible,
        T.RawValue == T.RawValue.ConvertedType>(field: String, defaultValue: T? = nil) -> [T]?
    {
        return try? self.from(field, defaultValue: defaultValue)
    }

[keith]

This is super super super late, but I just merged #125 with support for this. Sorry about that.