Fixed bug #50224 where float without decimals were converted to integer when encode to JSON

Refs #635
@datibbaw I was using the same constant used internally in PHP before (see [spprintf.c](https://github.com/php/php-src/blob/af6c11c5f060870d052a2b765dc634d9e47d0f18/main/spprintf.c#L118-L126)), but @bukka mentioned it is too big for a number and he was using this new number on the ext-jsond extension.

I can use the `DBL_*` constants, but I am not very familiar with those and I don't know how it works in 32/64bits systems. This is another reason to leave this constant.