Fix a bug in BaseDimensions.ToString() (#1431)

Current implementation of `AppendDimensionString` ignores value `-1`.
This PR fixes the bug.

---------

Co-authored-by: ygorshkov <yury.gorshkov@wartsila.com>

Author: ygorshkov

UnitsNet.Tests/BaseDimensionsTests.cs

@@ -695,7 +695,13 @@ public void CheckBaseDimensionMultiplicationWithForceEqualsMassTimesAcceleration
         [Fact]
         public void CheckToStringUsingMolarEntropy()
         {
-            Assert.Equal("[Length]^2[Mass][Time]^-2[Temperature][Amount]", MolarEntropy.BaseDimensions.ToString());
+            Assert.Equal("[Length]^2[Mass][Time]^-2[Temperature]^-1[Amount]^-1", MolarEntropy.BaseDimensions.ToString());
+        }
+
+        [Fact]
+        public void CheckToStringUsingSpeed()
+        {
+            Assert.Equal("[Length][Time]^-1", Speed.BaseDimensions.ToString());
         }
 
         [Fact]

UnitsNet/BaseDimensions.cs

@@ -182,13 +182,11 @@ public override string ToString()
 
         private static void AppendDimensionString(StringBuilder sb, string name, int value)
         {
-            var absoluteValue = Math.Abs(value);
-
-            if(absoluteValue > 0)
+            if (0 != value)
             {
                 sb.AppendFormat("[{0}]", name);
 
-                if(absoluteValue > 1)
+                if (1 != value)
                     sb.AppendFormat("^{0}", value);
             }
         }