[BUG] Reciprocal of a negative fraction has invalid state

[lipchev]

Consider the following scenario:

	Name	Value	Type
	new Fraction(-1, 10).Reciprocal().ToDecimal()	-10	decimal

	Expression	Value	Type
	new Fraction(-1, 10).Reciprocal().IsEquivalentTo(new Fraction(-10))	false	bool

Normally this should have been true for the Equals as well, but I think this illustrates the issue better.

This problem stems from the fact that the sign of the fraction is not handled here

    public static Fraction Reciprocal(Fraction fraction) =>
        new Fraction(fraction.Denominator, fraction.Numerator, fraction.State);

According to GetReducedFraction a Normalized fraction should have it's signs normalized:

            if (denominator.Sign == -1) {
                // Denominator must not be negative after normalization
                numerator = BigInteger.Negate(numerator);
                denominator = BigInteger.Negate(denominator);
            }

I'd be willing to create a PR, but I was hoping that we could merge my "cleanup" of the test project first, if you don't mind..

[danm-de]

Oh, thanks.

I didn't even think about that when I merged #21 :-)