Handle IConvertible case to return specific type FluidValues

[atthevergeof]

fixes #609

The JValue class does implement IConvertible, but in Fluid.Create, when System.Type.GetTypeCode() is called with JValue type in the argument, it returns TypeCode.Object. And under this case, it was going to the IFormattable case and being converted to string regardless of the underlying data type. But when JValue is cast to IConvertible and then the GetTypeCode() method is called on the object, it does return the correct type code corresponding to the underlying value.

So to resolve the issue #609, I have moved the IConvertible case above IFormattable, and also changed its implementation to extract the underlying value and call the Create method again with that value. To prevent any infinite recursion, I have explicitly handled the TypeCode.Object case by returning the string value for it.

[lahma]

You probably want to add some test cases to demonstrate what you have fixed and also to ensure nobody breaks the behavior you are after later on.

[atthevergeof]

Added a test case to validate the change