Fix double to microseconds conversion in ext/date.

[ranvis]

As mentioned by @kylekatarnls in #14332 (comment), php_date_initialize_from_ts_double() doesn't handle the fractional part of a double in the way users would expect.

var_dump(DateTime::createFromTimestamp(1599828571.235612)); // => ...235611

Also for example, 1.000_001 (1.000_000_99999999991773...) is transformed to @1.000000.

The updated code calculates the delta between the provided value and the next double value (in the case above, 1.000_001_00000000013978...), and if the delta is less than or equal to 0.5 microseconds, it uses the next value.
I think this solves the problem while keeping the original results on other cases.

CC: @marc-mabe

[ranvis]

Closing as per #14375.