Add writeUnlessDefault function

[neoeinstein]

There are times, especially with option types, where it is nice to be able to omit default values (that would otherwise be output as null). Json.writeUnlessDefault provides a nice complimentary function to Json.readOrDefault, which can be used to provide a default value if the expected value is omitted.

Tests included.

[kolektiv]

Ah yeah I think I see how this fits and makes sense. Cool, ok, I'll merge this in to dev, sort some internal tweaks out and push a new release out shortly. Thanks!