Ensure defaultValue is always applied in the case of nulls

[rayners]

I noticed that the default and defaultValue properties are applied from functions in the case of null or undefined (see https://github.com/zemirco/json2csv/blob/v4/lib/JSON2CSVBase.js#L81), but only undefined for simple properties (https://github.com/zemirco/json2csv/blob/v4/lib/utils.js#L4). I also noticed that the tests for defaultValue were accidentally relying on the null -> "" transformation, so I updated them to exercise the behavior better.

[coveralls]

Coverage remained the same at 100.0% when pulling 2b94f28 on rayners:ensure-null-gets-defaultValue into d891c64 on zemirco:v4.

[knownasilya]

Awesome, love when tests are submitted with a fix!

[knownasilya]

Released as v4 patch

[juanjoDiaz]

A bit late to the party but, I have a couple comments/questions:

null is not the same as undefined so I'm not 100% sure if we should consider it as undefined. But that's ok.

More importantly, what is the reason to change the tests to use a default value of '-' instead of ''?