Fixed issue with invalid json due to skipped field

Fixed issue with culture representation of data when serializing floating point types

Added NetJSON.SkipDefaultValue = (default|true) to turn off skipping of default value such as int = 0 or string = null

Fixed https://github.com/rpgmaker/NetJSON/issues/18

Signed-off-by: rpgmaker <olamide.bakre@gmail.com>