Added unit test for Horizontal and Vertical resolution.

src/ImageSharp/MetaData/ImageMetaData.cs

@@ -79,7 +79,7 @@ namespace ImageSharp
 
             set
             {
-              if (value >= 0)
+              if (value > 0)
               {
                   this.horizontalResolution = value;
               }
@@ -100,7 +100,7 @@ namespace ImageSharp
 
             set
             {
-                if (value >= 0)
+                if (value > 0)
                 {
                     this.verticalResolution = value;
                 }

tests/ImageSharp.Tests/MetaData/ImageMetaDataTests.cs

@@ -39,6 +39,38 @@ namespace ImageSharp.Tests
             Assert.Equal(1, clone.RepeatCount);
         }
 
+        [Fact]
+        public void HorizontalResolution()
+        {
+            ImageMetaData metaData = new ImageMetaData();
+            Assert.Equal(96, metaData.HorizontalResolution);
+
+            metaData.HorizontalResolution=0;
+            Assert.Equal(96, metaData.HorizontalResolution);
+
+            metaData.HorizontalResolution=-1;
+            Assert.Equal(96, metaData.HorizontalResolution);
+
+            metaData.HorizontalResolution=1;
+            Assert.Equal(1, metaData.HorizontalResolution);
+        }
+
+        [Fact]
+        public void VerticalResolution()
+        {
+            ImageMetaData metaData = new ImageMetaData();
+            Assert.Equal(96, metaData.VerticalResolution);
+
+            metaData.VerticalResolution = 0;
+            Assert.Equal(96, metaData.VerticalResolution);
+
+            metaData.VerticalResolution = -1;
+            Assert.Equal(96, metaData.VerticalResolution);
+
+            metaData.VerticalResolution = 1;
+            Assert.Equal(1, metaData.VerticalResolution);
+        }
+
         [Fact]
         public void SyncProfiles()
         {