Fix Gen.choose for Double infinity

[ashawley]

I was able to verify #379 with the following test that checks that there is at least one non-zero value in a sample of three:

    import Double.{NegativeInfinity, PositiveInfinity}
    property("choose-infinite-double") = {
      Prop.forAllNoShrink(listOfN(3, choose(NegativeInfinity, PositiveInfinity))) { xs =>
        xs.exists(_ != 0d)
      }
    }

Here it is failing currently:

[info] ! Gen.choose-infinite-double: Falsified after 0 passed tests.
[info] > ARG_0: List("0.0", "0.0", "0.0")

The test assumes that flipping a Double.MaxValue-sided coin and getting a zero three times in a row is impossible. Someone may need to check my math on that.

scala> 2 * math.pow(Long.MaxValue.toDouble, -3d)
res0: Double = 2.5489470578119236E-57

The fix here just substitutes Double.MinValue for Double.NegativeInfinity and Double.MaxValue for Double.PositiveInfinity in the internal function Gen.Choose.chDbl, and also adds the special constants Double.NegativeInfinity and Double.PositiveInfinity at a frequency of 10% (for both respectively).