Fix DoubleField failing with JInt

Right now, any JSON value without a decimal value will fail when you try to put it into a DoubleField.  For example:

{ "value": 1234 } becomes JField( "value", JInt(1234) ), which in turn makes DoubleField very upset because it doesn't know what to do with JInt. 

I added a case for JInt in DoubleField's setFromJValue and added an associated test case.

In a separate commit I did the opposite for LongField/IntField so they can take JDoubles.