Removing the redundant requirement of having "fields property with empty array"

Thanks @JamesNK for coming out with an excellent support for JSON Serialization & Deserialization of F# Discriminated Union. 

In FSharp, Values are optionals in the [Discriminated Union] (http://msdn.microsoft.com/en-us/library/vstudio/dd233226(v=vs.100).aspx). But In JSON.NET library we are forced to have an empty "Fields" array in order to serialize the discriminated unions with empty values.

IMHO, it is redundant and we can relax this requirement. This pull request would solve this by removing this constraint.

*Before Change*
![before change](https://cloud.githubusercontent.com/assets/1128916/4368553/81abca68-42ef-11e4-8087-4cfcd5d134bd.png)
*After Change*
![after change](https://cloud.githubusercontent.com/assets/1128916/4368557/877ec710-42ef-11e4-83f5-d0ab2460b492.png)