Numeric field formatter modifies negative values

This patch fixes the numeric field formatter when handling negative values. Under certain circumstances, the following code can alter the value of a negative numeric value rather than simply adjust its display:

$value = number_format(floor($value), 0, '', $this->options['separator']);

If $value is -109.3, floor() will return -110, not -109 as desired.

This patch fixes this issue by adding a check for whether the value in question is positive or negative and using ceil() for negative values.