java client should ignore null fields by default

[fehguy]

From https://groups.google.com/forum/#!topic/swagger-swaggersocket/EnEURydhPZA, we may want to disable serializing null values in the JSON.java.

[fehguy]

@wing328 i'm inclined to do this, any concerns?

[wing328]

@fehguy no concern from me. Ruby, PHP, Python, etc are already behaving this way (skip null value). We missed the implementation for Java templates and will submit a PR to change the behaviour to skip null value. Sorry.

[fehguy]

No problem at all! Thanks for the pr