Write fill attribute value if it differs from parent element

If elements are nested in groups and have different fill values than
their containing group the fill value wouldn't be written correctly.

Source/Painting/SvgColourServer.cs

@@ -61,5 +61,13 @@ namespace Svg
 
 		}
 
+        public override bool Equals(object obj)
+        {
+            var objColor = obj as SvgColourServer;
+            if (objColor == null)
+                return false;
+
+            return this.Colour.ToArgb() == objColor.Colour.ToArgb();
+        }
     }
 }

Source/SvgElement.cs

@@ -347,17 +347,30 @@ namespace Svg
                 {
                     object propertyValue = attr.Property.GetValue(this);
 
+                    var forceWrite = false;
+                    if ((attr.Attribute.Name == "fill") && (Parent != null))
+                    {
+                        var parentValue = ResolveParentAttributeValue(attr.Attribute.Name);
+                        if (parentValue != null)
+                        {
+                            if (parentValue.Equals(propertyValue))
+                                continue;
+
+                            forceWrite = true;
+                        }
+                    }
+
                     if (propertyValue != null)
                     {
                         var type = propertyValue.GetType();
                         string value = (string)attr.Property.Converter.ConvertTo(propertyValue, typeof(string));
 
-                        if (!SvgDefaults.IsDefault(attr.Attribute.Name, value))
+                        if (!SvgDefaults.IsDefault(attr.Attribute.Name, value) || forceWrite)
                         {
                             writer.WriteAttributeString(attr.Attribute.NamespaceAndName, value);
                         }
                     }
-                    else if(attr.Attribute.Name == "fill") //if fill equals null, write 'none'
+                    else if (attr.Attribute.Name == "fill") //if fill equals null, write 'none'
                     {
                         string value = (string)attr.Property.Converter.ConvertTo(propertyValue, typeof(string));
                         writer.WriteAttributeString(attr.Attribute.NamespaceAndName, value);
@@ -370,7 +383,27 @@ namespace Svg
             {
                 writer.WriteAttributeString(item.Key, item.Value);
             }
+        }
+
+        private object ResolveParentAttributeValue(string attributeKey)
+        {
+            attributeKey = char.ToUpper(attributeKey[0]) + attributeKey.Substring(1);
+
+            object parentValue = null;
+
+            var currentParent = Parent;
+            while (currentParent != null)
+            {
+                if (currentParent.Attributes.ContainsKey(attributeKey))
+                {
+                    parentValue = currentParent.Attributes[attributeKey];
+                    if (parentValue != null)
+                        break;
+                }
+                currentParent = currentParent.Parent;
+            }
 
+            return parentValue;
         }
 
         protected virtual void Write(XmlTextWriter writer)