Fix MathContract<decimal>.Exp().

Fix MathContract<decimal>.Exp().

double->decimal conversion can be "lossy"; consequently, this was failing
under .NET (but not mono).

	Assert.AreEqual (m.E, m.Exp (m.FromInt32 (1)));

Apparently the 'double->decimal' conversion operator doesn't invoke the
decimal(double) constructor under .NET; it does under Mono.  Whew knew?

Changing DecimalMath to use the decimal(double) constructor instead of the
(decimal) cast operator allows the tests to pass under .NET.