toBigInt should round identically for Double and BigDecimal (backport #3921)

[mergify]

This is an automatic backport of pull request #3921 done by Mergify.

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Original PR Body

This very minor PR fixes a bug in Num.toBigInt(x: BigDecimal, binaryPoint: Int).

The current function gives the impression to round the input BigDecimal:

val result = (x * multiplier).rounded.toBigInt

However:

BigDecimal(1.4).rounded.toBigInt // : BigInt = 1
BigDecimal(1.5).rounded.toBigInt // : BigInt = 1
BigDecimal(1.6).rounded.toBigInt // : BigInt = 1

I'm assuming this is not the intended behaviour.

The underlying issue is with the rounded function which uses the BigDecimal's MathContext. I changed this to use setScale(0) instead.

I added a testcase for rounding positive numbers. I would've added a testcase for negative ones, but as far as I know there is no RoundingMode that replicates the rounding of Doubles there (round half to positive infinity).

Contributor Checklist

• Did you add Scaladoc to every public function/method?
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Type of Improvement

• Bugfix

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