Precision: fix Decrement with negative count

Fixes #904

Author: cdrnet

src/Numerics.Tests/PrecisionTest.cs

@@ -190,6 +190,32 @@ public void DecrementAtZero()
             Assert.AreEqual(-2 * double.Epsilon, x);
         }
 
+        /// <summary>
+        /// Increment at zero.
+        /// </summary>
+        [Test]
+        public void IncrementDecrementWithCount()
+        {
+            var x = -2 * double.Epsilon;
+            Assert.AreEqual(-2 * double.Epsilon, x);
+            x = x.Increment(2);
+            Assert.AreEqual(0.0, x);
+            x = x.Increment(2);
+            Assert.AreEqual(2 * double.Epsilon, x);
+            x = x.Decrement(2);
+            Assert.AreEqual(0.0, x);
+            x = x.Decrement(2);
+            Assert.AreEqual(-2 * double.Epsilon, x);
+            x = x.Decrement(-2);
+            Assert.AreEqual(0.0, x);
+            x = x.Decrement(-2);
+            Assert.AreEqual(2 * double.Epsilon, x);
+            x = x.Increment(-2);
+            Assert.AreEqual(0.0, x);
+            x = x.Increment(-2);
+            Assert.AreEqual(-2 * double.Epsilon, x);
+        }
+
         /// <summary>
         /// Increment at min/max.
         /// </summary>

src/Numerics/Precision.cs

@@ -319,7 +319,7 @@ public static double Decrement(this double value, int count = 1)
 
             if (count < 0)
             {
-                return Decrement(value, -count);
+                return Increment(value, -count);
             }
 
             // Translate the bit pattern of the double to an integer.