float/double fields converted to string by xtypes_to_json when 0

[mac-apt]

In this function float/double types get judged whether they should be converted to numerical or string JSON field according to result of std::isnormal. That causes zeroes to be converted to string also, and I don't see any reason for that. Is that a bug?

[mac-apt]

I've made a patch correcting this issue, feel free to use it.
zero_float.patch