IQuantity As/ToUnit method exceptions (#621)

* IQuantity As/ToUnit methods would allow casting an Enum to the wrong unit type. Throw exception.

* Pretty :)

Author: tmilnthorp

UnitsNet.Tests/IQuantityTests.cs

@@ -0,0 +1,26 @@
+// Licensed under MIT No Attribution, see LICENSE file at the root.
+// Copyright 2013 Andreas Gullberg Larsen ([email protected]). Maintained at https://github.com/angularsen/UnitsNet.
+
+using System;
+using UnitsNet.Units;
+using Xunit;
+
+namespace UnitsNet.Tests
+{
+    public class IQuantityTests
+    {
+        [Fact]
+        public void As_GivenWrongUnitType_ThrowsArgumentException()
+        {
+            IQuantity length = Length.FromMeters(1.2345);
+            Assert.Throws<ArgumentException>(() => length.As(MassUnit.Kilogram));
+        }
+
+        [Fact]
+        public void ToUnit_GivenWrongUnitType_ThrowsArgumentException()
+        {
+            IQuantity length = Length.FromMeters(1.2345);
+            Assert.Throws<ArgumentException>(() => length.ToUnit(MassUnit.Kilogram));
+        }
+    }
+}