Fix sign and padding in string format for negative Numeric

[zuckschwerdt]

This fixes the sign and padding problems in string format for negative Numerics.

• The decimal part must not be negative when formatting to string, change to use .abs().
• The integer part must have a "signed zero" when the value is negative, change to use .abs() and explicit sign.

I.e. before / after

• Numeric(-123, 1) was "-12.-3" now is "-12.3"
• Numeric(-123, 2) was "-1.-23" now is "-1.23"
• Numeric(-123, 3) was "0.-123" now is "-0.123"
• Numeric(-123, 4) was "0.-123" now is "-0.0123"

Fixes #368