Force to show decimal in CommafWithDigits

[zeing]

My float64 number is 1000.00

Actual:
CommafWithDigits(1000.00, 2) => 1,000

Expect:
CommafWithDigits(1000.00, 2) => 1,000.00

When inputting a decimal like 2 it should have xxxx.xx

[sillyguodong]

I also think that this function should always return a string with the specified precision.