allow setting the profile header

src/ImageSharp/MetaData/Profiles/ICC/IccProfile.cs

@@ -49,7 +49,7 @@ namespace ImageSharp
         }
 
         /// <summary>
-        /// Gets the profile header
+        /// Gets or sets the profile header
         /// </summary>
         public IccProfileHeader Header
         {
@@ -58,6 +58,11 @@ namespace ImageSharp
                 this.InitializeHeader();
                 return this.header;
             }
+
+            set
+            {
+                this.header = value;
+            }
         }
 
         /// <summary>