Math errors with float -> int implicit fleece conversion

To save space, fleece was converting floats with no decimal information (e.g. 5.0) to integers. However, this had the side effect of causing math errors if both sides an an operation in a query (e.g. 5.0 / 15.0) had no decimal information because 5.0 / 15.0 != 5 / 15. Unfortunately this means saving the extra bytes on disk even when they are not needed to save the logical value.